Reduce allocations during variable coercion

[michaelstaib]

No description provided.

[Copilot]

Pull request overview

This PR optimizes JSON variable coercion in Fusion execution to reduce allocations, while adding regression tests to ensure error paths, depth limits, OneOf validation, and numeric lexeme preservation remain correct.

Changes:

• Reworks JsonVariableCoercion to defer path construction via an inline path-segment buffer and only materialize Path/string representations on error.
• Avoids number-token string/byte allocations by reading raw UTF-8 number lexemes via JsonMarshal.GetRawUtf8Value.
• Adds tests covering nested variable error paths, OneOf error behavior, max-depth boundary enforcement, and numeric lexeme/format preservation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/HotChocolate/Fusion/test/Fusion.Execution.Tests/Execution/VariableCoercionHelperTests.cs	Adds regression tests for nested paths, OneOf, max depth, and numeric lexeme preservation.
src/HotChocolate/Fusion/src/Fusion.Execution/Execution/JsonVariableCoercion.cs	Implements allocation reductions via deferred path segments and raw UTF-8 numeric parsing.

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[github-actions]

Patch coverage

90.4% of changed lines covered (85/94)

File	Covered	Changed	Patch %
…/src/Fusion.Execution/Execution/JsonVariableCoercion.cs	85	94	90.4% 🟡

Uncovered changed lines (JSON)

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Project coverage: 53.7% (230153/428802 lines)