[WIP] Fix failing GitHub Actions job build-test-pack

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Fix the failing GitHub Actions job "build-test-pack"
Analyze the Actions logs, identify the root cause of the failure, and implement a fix.
Check run ID: 82531174580
Job URL: https://github.com/jongalloway/MarpToPptx/actions/runs/27889779720/job/82531174580

[Copilot]

Pull request overview

This PR addresses the failing CI job build-test-pack by hardening SyntaxHighlighter against non-thread-safe behavior in TextMateSharp that was causing SyntaxHighlighterTests.Tokenize_CSharp_ProducesColoredRunsForKeywords to fail with a NullReferenceException.

Changes:

• Serialize TextMateSharp registry/grammar usage by expanding the existing lock to cover both grammar loading and tokenization.
• Add a defensive null-check around grammar.TokenizeLine(...) to prevent NullReferenceException when a null result is returned.
• Update the inline comment to document the broader thread-safety constraints (registry + grammar instances).

Show a summary per file

File	Description
src/MarpToPptx.Pptx/Rendering/SyntaxHighlighter.cs	Serializes TextMateSharp operations and adds a null-guard to avoid CI test failures due to unexpected null tokenization results.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 1