Fix peek issue (#44456)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request
This pull request focuses on improving the security and behavior of
Markdown file previews. The main changes include disabling HTML
rendering in Markdown to prevent potential security issues, and ensuring
that the `IsDevFilePreview` flag is set correctly when previewing
Markdown files.

**Markdown rendering and preview behavior:**

* Disabled HTML rendering in the Markdown pipeline by adding
`.DisableHtml()` in `MarkdownHelper.MarkdownHtml`, which helps prevent
XSS and other security issues in file previews.
* Explicitly set `IsDevFilePreview` to `false` when handling Markdown
files in `WebBrowserPreviewer`, ensuring correct preview state.

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## PR Checklist

- [ ] Closes: #xxx
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **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
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **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

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/common/FilePreviewCommon/MarkdownHelper.cs

@@ -39,7 +39,7 @@ namespace Microsoft.PowerToys.FilePreviewCommon
             var softlineBreak = new Markdig.Extensions.Hardlines.SoftlineBreakAsHardlineExtension();
 
             MarkdownPipelineBuilder pipelineBuilder;
-            pipelineBuilder = new MarkdownPipelineBuilder().UseAdvancedExtensions().UseEmojiAndSmiley().UseYamlFrontMatter().UseMathematics();
+            pipelineBuilder = new MarkdownPipelineBuilder().UseAdvancedExtensions().UseEmojiAndSmiley().UseYamlFrontMatter().UseMathematics().DisableHtml();
             pipelineBuilder.Extensions.Add(extension);
             pipelineBuilder.Extensions.Add(softlineBreak);
 

src/modules/peek/Peek.FilePreviewer/Previewers/WebBrowserPreviewer/WebBrowserPreviewer.cs

@@ -130,6 +130,7 @@ namespace Peek.FilePreviewer.Previewers
                     }
                     else if (isMarkdown)
                     {
+                        IsDevFilePreview = false;
                         var raw = await ReadHelper.Read(File.Path.ToString());
                         Preview = new Uri(MarkdownHelper.PreviewTempFile(raw, File.Path, TempFolderPath.Path));
                     }