Reduce allocations and telemetry overhead in Fusion execution

[michaelstaib]

No description provided.

[Copilot]

Pull request overview

This PR reduces per-node allocation and telemetry overhead in the Fusion execution pipeline by making ExecutionNodeResult a value type and short-circuiting telemetry capture when telemetry collection is disabled.

Changes:

• Convert ExecutionNodeResult from a reference type to a readonly record struct to avoid allocations.
• Guard activity/timing capture in ExecutionNode.ExecuteAsync behind context.CollectTelemetry.
• Update ExecutionState.TryDequeueCompletedResult to return a non-nullable ExecutionNodeResult, and add focused execution-state/telemetry unit tests.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/HotChocolate/Fusion/test/Fusion.Execution.Tests/Execution/ExecutionNodeCompletionTests.cs	Adds coverage for telemetry gating, single-completion behavior across outcomes, dependent scheduling, and merge-failure application with struct results.
src/HotChocolate/Fusion/src/Fusion.Execution/Execution/Nodes/ExecutionNodeResult.cs	Switches ExecutionNodeResult to a readonly record struct to reduce allocations.
src/HotChocolate/Fusion/src/Fusion.Execution/Execution/Nodes/ExecutionNode.cs	Skips Activity.Current and timestamp work when telemetry is disabled.
src/HotChocolate/Fusion/src/Fusion.Execution/Execution/ExecutionState.cs	Adjusts dequeue API to match the non-nullable struct result type.

Comments suppressed due to low confidence (1)

src/HotChocolate/Fusion/src/Fusion.Execution/Execution/Nodes/ExecutionNode.cs:122

• When telemetry collection is disabled, GetTransportDetails will always return (null, null) but this still performs an extra method call and redundant CollectTelemetry check. Since you already have collectTelemetry here, you can avoid the call entirely on the hot path when telemetry is off.

                error,
                context.GetDependentsToExecute(this),
                context.GetSkippedDefinitions(this),
                context.GetVariableValueSets(this),
                context.GetTransportDetails(this));

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Patch coverage

100.0% of changed lines covered (4/4)

File	Covered	Changed	Patch %
…/Fusion/src/Fusion.Execution/Execution/Nodes/ExecutionNode.cs	4	4	100.0% 🟢

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Project coverage: 53.4% (227760/426465 lines)