Add errors and warnings for closed hierarchies

[BillWagner]

Fixes #54016

Pick up a few outstanding union based errors as well.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-messages/pattern-matching-warnings.md	Resolve errors and warnings in pattern matching expressions
docs/csharp/language-reference/compiler-messages/union-declaration-errors.md	"Resolve errors and warnings related to union type and closed hierarchy declarations"
docs/csharp/language-reference/toc.yml	docs/csharp/language-reference/toc
docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md	Coming in C# 15

[Copilot]

Pull request overview

This PR updates the C# compiler diagnostics documentation to cover new closed hierarchy errors and consolidates a few remaining union-related diagnostics into the relevant reference pages.

Changes:

• Updates the union diagnostics article to include closed hierarchy diagnostics (CS9380–CS9384) and additional union-member-provider diagnostics (CS9385–CS9387).
• Moves CS9372 indexing to the pattern matching diagnostics page and updates the C# language reference TOC metadata accordingly.
• Removes union/closed-hierarchy error codes from the generic “no specifics” fallback page now that dedicated coverage exists.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md	Removes union/closed-hierarchy codes from the fallback keyword list.
docs/csharp/language-reference/toc.yml	Renames the union entry and updates diagnostic keyword indexing in TOC metadata.
docs/csharp/language-reference/compiler-messages/union-declaration-errors.md	Expands coverage to include closed hierarchy and union-member-provider diagnostics.
docs/csharp/language-reference/compiler-messages/pattern-matching-warnings.md	Adds CS9372 indexing and guidance in the pattern matching diagnostics page.