Add RGB hex color preview to Advanced Paste clipboard history (#43990)

<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
-->
## Summary of the Pull Request

This pull request adds support for recognizing and displaying clipboard
items that are valid RGB hex color codes (such as `#FFBFAB` or `#abc`)
in the Advanced Paste module. It introduces logic to detect hex color
strings, converts them to color values, and updates the UI to show a
color preview for these items. The changes also include comprehensive
unit tests for the new functionality.

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## PR Checklist

- [x] Closes: #43538
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [x] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [x] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
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- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

**Clipboard color detection and conversion:**

* Added `ClipboardItemHelper.IsRgbHexColor` method using a compiled
regex to identify valid hex color strings in clipboard text.
[[1]](diffhunk://#diff-7429196ad30cd0bce57b102669da4dc13d43a09579e99ceac7cc0f7dc101cd2bR62-R86)
[[2]](diffhunk://#diff-7429196ad30cd0bce57b102669da4dc13d43a09579e99ceac7cc0f7dc101cd2bR112-R114)
* Introduced `HexColorConverterHelper.ConvertHexColorToRgb` utility to
convert hex color strings to `Windows.UI.Color`, handling both 3-digit
and 6-digit formats.

**UI enhancements for color previews:**

* Updated `ClipboardHistoryItemPreviewControl` to include a color
preview grid that displays an ellipse filled with the detected color and
the color code as text, using the new `HexColorToBrushConverter`.
[[1]](diffhunk://#diff-2ed6014d4c17037b9cd0ab397e40b9069b1e7fe47a700673f34e8217d78124d5R29-R48)
[[2]](diffhunk://#diff-2ed6014d4c17037b9cd0ab397e40b9069b1e7fe47a700673f34e8217d78124d5R14)
[[3]](diffhunk://#diff-0c26c92697f6bb38fa40160fc8b18f0876ddc8d828a510034411001aa2e05063R1-R28)
* Modified logic in `ClipboardHistoryItemPreviewControl.xaml.cs` to
ensure color previews are shown only for detected color items and to
adjust visibility of text and glyph previews accordingly.

**Unit tests for color detection and conversion:**

* Added unit tests for hex color conversion
(`HexColorToColorConverterTests.cs`) and color detection logic
(`ClipboardItemHelperTests.cs`) to verify correct behavior for valid,
invalid, and edge-case inputs.
[[1]](diffhunk://#diff-d81d997d5fb414f1563c31c38681113aaa9c847ef05bb77662d30bd1310d6b8eR1-R61)
[[2]](diffhunk://#diff-185e8954ca6f061bf5d60d0c61ac6cfd87bd1a48ebda11a8172e3496a050fe85R1-R36)

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed
* Copied to the clipboard a color encoded text like: #FFBFAB
* Opened Advanced Paste and noticed the color: 

<img width="467" height="309" alt="image"
src="https://github.com/user-attachments/assets/6cedce89-9833-4efb-abf9-3cfe8e8f32f0"
/>

---------

Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>
Co-authored-by: crramirez <8397379+crramirez@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/modules/AdvancedPaste/AdvancedPaste.UnitTests/ConvertersTests/HexColorToColorConverterTests.cs

@@ -0,0 +1,56 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using AdvancedPaste.Converters;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Windows.UI;
+
+namespace AdvancedPaste.UnitTests.ConvertersTests;
+
+[TestClass]
+public sealed class HexColorToColorConverterTests
+{
+    [TestMethod]
+    public void TestConvert_ValidSixDigitHex_ReturnsColor()
+    {
+        Color? result = HexColorConverterHelper.ConvertHexColorToRgb("#FFBFAB");
+        Assert.IsNotNull(result);
+
+        var color = (Windows.UI.Color)result;
+        Assert.AreEqual(255, color.R);
+        Assert.AreEqual(191, color.G);
+        Assert.AreEqual(171, color.B);
+        Assert.AreEqual(255, color.A);
+    }
+
+    [TestMethod]
+    public void TestConvert_ValidThreeDigitHex_ReturnsColor()
+    {
+        Color? result = HexColorConverterHelper.ConvertHexColorToRgb("#abc");
+        Assert.IsNotNull(result);
+
+        var color = (Windows.UI.Color)result;
+
+        // #abc should expand to #aabbcc
+        Assert.AreEqual(170, color.R); // 0xaa
+        Assert.AreEqual(187, color.G); // 0xbb
+        Assert.AreEqual(204, color.B); // 0xcc
+        Assert.AreEqual(255, color.A);
+    }
+
+    [TestMethod]
+    public void TestConvert_NullOrEmpty_ReturnsNull()
+    {
+        Assert.IsNull(HexColorConverterHelper.ConvertHexColorToRgb(null));
+        Assert.IsNull(HexColorConverterHelper.ConvertHexColorToRgb(string.Empty));
+        Assert.IsNull(HexColorConverterHelper.ConvertHexColorToRgb("   "));
+    }
+
+    [TestMethod]
+    public void TestConvert_InvalidHex_ReturnsNull()
+    {
+        Assert.IsNull(HexColorConverterHelper.ConvertHexColorToRgb("#GGGGGG"));
+        Assert.IsNull(HexColorConverterHelper.ConvertHexColorToRgb("#12345"));
+    }
+}

src/modules/AdvancedPaste/AdvancedPaste.UnitTests/HelpersTests/ClipboardItemHelperTests.cs

@@ -0,0 +1,36 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using AdvancedPaste.Helpers;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace AdvancedPaste.UnitTests.HelpersTests;
+
+[TestClass]
+public sealed class ClipboardItemHelperTests
+{
+    [TestMethod]
+    [DataRow("#FFBFAB", true)]
+    [DataRow("#000000", true)]
+    [DataRow("#FFFFFF", true)]
+    [DataRow("#fff", true)]
+    [DataRow("#abc", true)]
+    [DataRow("#123456", true)]
+    [DataRow("#AbCdEf", true)]
+    [DataRow("FFBFAB", false)] // Missing #
+    [DataRow("#GGGGGG", false)] // Invalid hex characters
+    [DataRow("#12345", false)] // Wrong length
+    [DataRow("#1234567", false)] // Too long
+    [DataRow("", false)]
+    [DataRow(null, false)]
+    [DataRow("  #FFF  ", true)] // Whitespace should be trimmed
+    [DataRow("Not a color", false)]
+    [DataRow("#", false)]
+    [DataRow("##FFFFFF", false)]
+    public void TestIsRgbHexColor(string input, bool expected)
+    {
+        bool result = ClipboardItemHelper.IsRgbHexColor(input);
+        Assert.AreEqual(expected, result, $"IsRgbHexColor(\"{input}\") should return {expected}");
+    }
+}