Respect document line endings in C# code actions and refactorings

[Copilot]

Several C# code actions were introducing hard-coded CRLF trivia into documents that otherwise used LF, which caused Linux runs to produce mixed line endings and forced a set of issue-tracked tests to stay disabled. This PR updates the affected generators/refactorings to follow the document/host newline convention instead of assuming Windows line endings.

• Shared newline generation

  • Switch shared C#/VB syntax services from hard-coded ElasticCarriageReturnLineFeed to elastic end-of-line trivia based on the current environment newline.
  • Update shared C# code-generation helpers that synthesize missing braces or insert generated members so they no longer inject CRLF by default.

• Direct C# fixer/refactoring call sites

  • Replace direct CRLF trivia construction in affected refactorings/code fixes, including:
    • split/merge if statements
    • convert if to switch
    • convert auto-property to full property
    • convert to record
    • extract method
    • introduce variable
  • Keep inserted blank lines and block formatting elastic, but no longer Windows-specific.

• Raw string conversion

  • Normalize embedded line endings when converting regular/interpolated strings to raw strings so generated source text follows the target document newline style.
  • Add a local helper for line-ending normalization compatible with the feature assembly targets.

• Issue-tracked tests

  • Re-enable the C# tests that were disabled specifically for this newline regression so they can validate LF/CRLF behavior again.

// Before
OpenBraceToken.WithLeadingTrivia(SyntaxFactory.ElasticCarriageReturnLineFeed)

// After
OpenBraceToken.WithLeadingTrivia(SyntaxFactory.ElasticEndOfLine(Environment.NewLine))

[CyrusNajmabadi]

this is weird, and shouldn't be necessary. by virtue of being elastic, the formatter should replace this with the right newline based on the options passed into it.

[JoeRobich]

I never identified where elastic newlines were supposed to be rewritten. I took at stab at adding it to the trivia rewriter (#83224) but haven't had time to review all the tests that broke due to changed formatting.