Fix Utf8MemoryBuilder state after sealing or abandoning

[michaelstaib]

No description provided.

[Copilot]

🟢 Approval recommended

The change is small and localized, and the added tests/snapshot directly validate the intended sealed/abandoned behavior and content preservation.

Pull request overview

This PR fixes Utf8MemoryBuilder’s internal state transitions so the builder is correctly treated as sealed after calling Seal() or Abandon(), preventing further use of write-related state (like NextIndex) while still preserving the sealed output.

Changes:

• Set _written = -1 after Seal() and Abandon() to consistently mark the builder as sealed/unusable.
• Add unit tests verifying NextIndex throws after sealing/abandoning and that sealing preserves previously written content.
• Add a Markdown snapshot to validate sealed content (WrittenSpan, WrittenMemory, and previously returned segment) remains consistent.

File summaries

File	Description
src/HotChocolate/Language/src/Language.Utf8/Utf8MemoryBuilder.cs	Marks the builder as sealed after Seal()/Abandon() by setting _written = -1.
src/HotChocolate/Language/test/Language.Tests/Parser/Utf8MemoryBuilderTests.cs	Adds regression tests for sealed/abandoned behavior and for content preservation after sealing.
src/HotChocolate/Language/test/Language.Tests/Parser/snapshots/Utf8MemoryBuilderTests.Seal_Should_PreserveWrittenContent_When_BufferContainsData.md	Adds snapshot coverage for preserved sealed content.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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Patch coverage

100.0% of changed lines covered (2/2)

File	Covered	Changed	Patch %
…/HotChocolate/Language/src/Language.Utf8/Utf8MemoryBuilder.cs	2	2	100.0% 🟢

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Project coverage: 57.9% (288209/497788 lines)