[Repo Assist] Fix #1383: Throw NotSupportedException for unrecognized CSS pseudo-classes

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Closes #1383

Root Cause

When HtmlDocument.CssSelect encounters a CSS pseudo-class or pseudo-element that is not implemented (e.g. :nth-child(1), :first-child, :not()), the tokenizer falls through to the generic catch-all case and throws a generic System.Exception with message "Invalid css selector syntax (char ':' at offset 0)". This message is misleading — the syntax is valid CSS, it's just not yet supported by FSharp.Data's CSS selector engine.

Fix

Added a specific case for the : character in the tokenizer (after all the recognised pseudo-class patterns) that reads the pseudo-class name and throws NotSupportedException with a clear, actionable message pointing to the documentation.

Before:

System.Exception: Invalid css selector syntax (char ':' at offset 0)
```

**After:**
```
System.NotSupportedException: CSS pseudo-class or pseudo-element ':nth-child' is not supported.
  See (fsprojects.github.io/redacted) for the list of supported selectors.

Trade-offs

• NotSupportedException is a more appropriate exception type than Exception for a known limitation.
• The error message now clearly distinguishes "unsupported but valid CSS" from "invalid CSS syntax".
• No functional behaviour changes for selectors that are already supported.

Test Status

• ✅ dotnet build succeeded (0 warnings relating to this change, 0 errors)
• ✅ All 30 existing HtmlCssSelectors tests pass
• ✅ Fantomas formatting check passed (dotnet fantomas --check)

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[github-actions]

✅ Pull request created: #1613

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🤖 This is an automated comment from Repo Assist.

CI Status Update

Linux ✅ passed — all tests pass on Ubuntu.

Windows ❌ failed — this failure is not caused by the changes in this PR. The failing test is:

XElement.Request includes default User-Agent header

The test is failing because the local ASP.NET Core Kestrel test server can't bind to localhost:49859 due to a Windows socket permission error (Error 10013 — WSAEACCES). This is an intermittent infrastructure issue on the Windows CI runner, unrelated to the CSS pseudo-class fix. The same failure appears on other PRs that also don't touch HTTP/socket code.

The fix in this PR (throwing NotSupportedException for unrecognised CSS pseudo-classes instead of Exception) is correct and the implementation is unaffected by this CI noise. A re-run of CI may produce a green result if the port is available, or a reviewer can approve based on the passing Linux run.

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