Convert the last hand-written C# example to a MarkdownSnippets snippet

[RaphaelFakhri]

Fixes #3413.

The conversion turned out to be nearly finished already. I inventoried every fence in every Markdown file in the repo, split by language and by whether it sits inside a <!-- snippet: --> region:

File	C# fences	Outside a snippet
docs/configure-and-customize-swaggergen.md	43	0
docs/configure-and-customize-annotations.md	13	0
docs/configure-and-customize-swaggerui.md	13	0
README.md	9	0
docs/configure-and-customize-redoc.md	8	0
docs/configure-and-customize-swagger.md	7	0
docs/configure-and-customize-cli.md	1	0
docs/migrating-to-v10.md	2	1

So the complete list of unconverted C# examples is one item: the ISchemaFilter.Apply example in docs/migrating-to-v10.md. package-readme.md, CONTRIBUTING.md and SECURITY.md have no fences, and AGENTS.md only has shell ones. The remaining non-C# fences are terminal, yaml, json, xml and one diff showing a public API signature change, none of which are compilable code.

That one example now comes from a new MigrationSchemaFilter type in DocumentationSnippets, so it compiles against the library like every other example.

One detail worth knowing, since no other snippet in the repo does this: the fence lives inside a bullet of the Migration Overview list. MarkdownSnippets preserves the indentation of the <!-- snippet: --> marker line, so an indented marker emits the anchor, the fence and the <sup> footer all indented to match and the bullet still renders as a single list item. If you would rather not depend on that, the alternative is restructuring the bullet so the example sits at column zero after the list, which is what every other snippet does. I left the code content identical, apart from the block's pre-existing mixed indentation and a trailing space on the closing brace, which the extraction normalises.

dotnet mdsnippets runs clean and is idempotent, markdownlint-cli2 against the repo's .markdownlint.json reports 0 issues, and DocumentationSnippets builds on net8.0, net9.0 and net10.0 with 0 warnings and 0 errors.

Separately, on #4127: the C# fences I added there can become guarded snippets by putting the preprocessor directives outside the begin and end markers, so the rendered page shows clean code while net8.0 skips the .NET 9 example. The two examples both declare a type named Priority, so they need separate files with distinct namespaces to keep the rendered text identical. Happy to do that in a follow-up if you want it done there too.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.21%. Comparing base (98c3501) to head (8570a6c).
⚠️ Report is 12 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4133   +/-   ##
=======================================
  Coverage   95.21%   95.21%           
=======================================
  Files         111      111           
  Lines        4115     4118    +3     
  Branches      843      844    +1     
=======================================
+ Hits         3918     3921    +3     
  Misses        197      197           

Flag	Coverage Δ
Linux	95.21% <ø> (+<0.01%)	⬆️
Windows	95.21% <ø> (+<0.01%)	⬆️
macOS	95.21% <ø> (+<0.01%)	⬆️

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