Fix color picker float format precision from 2 to 4 decimal places

Increase float color format precision to 4 decimal places so all
256 byte values (0-255) can be uniquely represented in the 0-1 float
range. With only 2 decimal places, only 101 unique values existed,
causing round-trip errors (e.g., byte 57 → 0.22 → 56.1 ≠ 57).

Changes:
- ColorFormatHelper.cs: Update 'f' format to "0.####" and 'F' to ".####"
- ColorRepresentationHelper.cs: Update ColorToFloat precision to 4
- Add test for non-trivial color that verifies precision fix

Agent-Logs-Url: https://github.com/microsoft/PowerToys/sessions/586aab56-475b-476e-b46a-7b18bea5a988

Co-authored-by: niels9001 <9866362+niels9001@users.noreply.github.com>

src/common/ManagedCommon/ColorFormatHelper.cs

@@ -394,8 +394,8 @@ namespace ManagedCommon
             { 'H', "X1" },      // hex uppercase one digit
             { 'x', "x2" },      // hex lowercase two digits
             { 'X', "X2" },      // hex uppercase two digits
-            { 'f', "0.##" },    // float with leading zero, 2 digits
-            { 'F', ".##" },     // float without leading zero, 2 digits
+            { 'f', "0.####" },  // float with leading zero, 4 digits
+            { 'F', ".####" },   // float without leading zero, 4 digits
             { 'p', "%" },       // percent value
             { 'i', "i" },       // int value
             { 's', "s" },       // string value

src/modules/colorPicker/ColorPickerUI.UnitTests/Helpers/ColorFormatConversionTest.cs

@@ -418,6 +418,16 @@ namespace ColorPicker.UnitTests.Helpers
             Assert.AreEqual(expected, result);
         }
 
+        [TestMethod]
+        public void GetStringRepresentation_VEC4_NonTrivialColor_HasSufficientPrecision()
+        {
+            // Color (237, 41, 57) should produce float values that round-trip back to the original byte values.
+            // With only 2 decimal places, 57/255 = 0.22 and 0.22*255 = 56.1, which is wrong.
+            // With 4 decimal places, 57/255 = 0.2235 and 0.2235*255 = 56.9925, which rounds to 57 correctly.
+            var result = ColorRepresentationHelper.GetStringRepresentation(Color.FromArgb(255, 237, 41, 57), "VEC4", ColorFormatHelper.GetDefaultFormat("VEC4"));
+            Assert.AreEqual("(0.9294f, 0.1608f, 0.2235f, 1f)", result);
+        }
+
         [TestMethod]
         public void GetStringRepresentation_EmptyFormat_ReturnsHex()
         {

src/modules/colorPicker/ColorPickerUI/Helpers/ColorRepresentationHelper.cs

@@ -107,8 +107,8 @@ namespace ColorPicker.Helpers
         private static string ColorToFloat(Color color)
         {
             var (red, green, blue) = ColorHelper.ConvertToDouble(color);
-            const int precision = 2;
-            const string floatFormat = "0.##";
+            const int precision = 4;
+            const string floatFormat = "0.####";
 
             return $"({Math.Round(red, precision).ToString(floatFormat, CultureInfo.InvariantCulture)}f"
                  + $", {Math.Round(green, precision).ToString(floatFormat, CultureInfo.InvariantCulture)}f"