[Fusion] Fix composition crash on unimplemented merged interface field

[glen-84]

Summary

• Composing two source schemas that declare the same-named interface with different fields crashed with KeyNotFoundException: The given key '…' was not present in the dictionary during post-merge validation.
• ImplementedByInaccessibleRule indexed type.Fields[interfaceField.Name] directly, assuming every implementing type provides every field of the merged interface. When a type implements the interface but is missing a field contributed by another subgraph, the lookup threw.
• Guard the lookup with TryGetField and skip absent fields; the unimplemented-field case is already reported by InterfaceFieldNoImplementationRule, which now surfaces a clear INTERFACE_FIELD_NO_IMPLEMENTATION error instead of an opaque crash.

Test plan

• Added ImplementedByInaccessibleRuleTests.Validate_MergedInterfaceFieldNotOnAllImplementers_Succeeds, which merges two subgraphs whose shared interface has disjoint fields (previously crashed).
• All 7 ImplementedByInaccessibleRuleTests pass, including the 6 existing @inaccessible detection cases.

[Copilot]

Pull request overview

Fixes a post-merge validation crash in Fusion composition when merged interface fields are not implemented by every implementing type, replacing a failing dictionary index with a guarded lookup so validation can continue and surface the intended INTERFACE_FIELD_NO_IMPLEMENTATION error from the appropriate rule.

Changes:

• Updated ImplementedByInaccessibleRule to use TryGetField and skip interface fields that aren’t present on the implementing type (instead of throwing KeyNotFoundException).
• Added a regression test ensuring ImplementedByInaccessibleRule validation does not crash when merged interfaces contain disjoint fields across subgraphs.

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/PostMergeValidationRules/ImplementedByInaccessibleRule.cs	Prevents crash by guarding field lookup on implementing types during interface-field iteration.
src/HotChocolate/Fusion/test/Fusion.Composition.Tests/PostMergeValidationRules/ImplementedByInaccessibleRuleTests.cs	Adds coverage for merged interface unions where implementers don’t all define every interface field.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.