[Fusion] Do not apply "InferKeysFromLookups" to invalid lookup fields

[glen-84]

Summary of the changes (Less than 80 chars)

• [Fusion] Do not apply "InferKeysFromLookups" to invalid lookup fields.

[Copilot]

Pull request overview

Updates Fusion source schema preprocessing to avoid inferring entity keys from lookup fields that are invalid per lookup constraints (e.g., no arguments, non-null return type, list return type).

Changes:

• Filter @lookup fields in InferKeysFromLookups() to exclude invalid lookup field shapes (no args / non-null / list).
• Add a regression test covering a mix of valid + invalid lookup fields to ensure only valid lookups contribute inferred @key directives.

Reviewed changes

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

File	Description
src/HotChocolate/Fusion/src/Fusion.Composition/SourceSchemaPreprocessor.cs	Excludes invalid @lookup fields from key inference to prevent incorrect @key directive application.
src/HotChocolate/Fusion/test/Fusion.Composition.Tests/SourceSchemaPreprocessorTests.cs	Adds coverage ensuring invalid lookup fields do not contribute to inferred keys.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (2732654) to head (8d09975).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff      @@
##   main   #9541   +/-   ##
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