Add parser cancellation plumbing

[marinasundstrom]

Summary

• thread cancellation tokens through parser contexts and honor them during token reads and recovery
• allow SyntaxTree.ParseText to enforce optional wall-clock timeouts and expose the timeout via SyntaxTreeProvider and the ravenc CLI
• document parser hardening progress around cancellation and timeouts

Testing

• dotnet build --property WarningLevel=0
• dotnet test test/Raven.CodeAnalysis.Tests /property:WarningLevel=0 -v minimal (fails: existing parser/runtime regressions and terminal logger crash after several known test failures)

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Codex Task