Flatten Fusion result selection set child lookups

[michaelstaib]

No description provided.

[Copilot]

Pull request overview

This PR simplifies Fusion’s ResultSelectionSet child-lookup implementation by removing the small/large subclass split and consolidating lookup strategy (linear scan vs dictionary) into a single sealed type, reducing virtual dispatch overhead while preserving the existing size threshold behavior.

Changes:

• Consolidated SmallResultSelectionSet and LargeResultSelectionSet behavior into a single sealed ResultSelectionSet with an optional dictionary-backed child lookup.
• Removed the two specialized selection set classes.
• Updated the benchmark to assert lookup strategy via a new UsesDictionaryLookup indicator instead of concrete subclass types.

Reviewed changes

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

File	Description
src/HotChocolate/Fusion/src/Fusion.Execution/Execution/Nodes/SmallResultSelectionSet.cs	Removed; behavior is now handled inside ResultSelectionSet.
src/HotChocolate/Fusion/src/Fusion.Execution/Execution/Nodes/LargeResultSelectionSet.cs	Removed; behavior is now handled inside ResultSelectionSet.
src/HotChocolate/Fusion/src/Fusion.Execution/Execution/Nodes/ResultSelectionSet.cs	Flattened child lookup logic into one sealed class with an optional dictionary for larger selection sets.
src/HotChocolate/Fusion/benchmarks/Fusion.Execution.Benchmarks/ResultSelectionSetLookupBenchmark.cs	Updated benchmark assertions to validate lookup strategy rather than relying on concrete types.

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[github-actions]

Patch coverage

92.6% of changed lines covered (88/95)

File	Covered	Changed	Patch %
…/src/Fusion.Execution/Execution/Nodes/ResultSelectionSet.cs	88	95	92.6% 🟡

Uncovered changed lines (JSON)

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}

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Project coverage: 53.8% (231723/430578 lines)