Fix Fusion require subtree routed to non-owning schema

[michaelstaib]

Fixes #10183

[Copilot]

Pull request overview

This PR tightens Fusion’s requirement inlining logic so that a requirement subtree is only folded into an existing planned fetch when the chosen source schema can actually resolve the full connector chain (fields + type conditions) down to the requirement target. This prevents the planner from generating subgraph documents that reference intermediate fields/fragments not declared by that subgraph.

Changes:

• Added a connector-chain resolvability check when selecting an ancestor step to inline requirements into (OperationPlanner).
• Added a regression test covering a case where the entry schema declares a nested lookup but does not own the required subtree (RequirementReentrancyTests).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/HotChocolate/Fusion/test/Fusion.Execution.Tests/Planning/RequirementReentrancyTests.cs	Adds a regression test and composed schema setup to ensure require subtrees stay in the owning subgraph even when another subgraph declares adjacent lookups.
src/HotChocolate/Fusion/src/Fusion.Execution/Planning/OperationPlanner.cs	Ensures ancestor-step requirement inlining only happens when the candidate schema can resolve the full connector chain (rejects invalid inlining into non-owning schemas).

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[github-actions]

Patch coverage

91.7% of changed lines covered (22/24)

File	Covered	Changed	Patch %
…/Fusion/src/Fusion.Execution/Planning/OperationPlanner.cs	22	24	91.7% 🟡

Uncovered changed lines (JSON)

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  "sha": "f8a71b9886136d67ed552b833eabdaec2582ed52",
  "files": [
    { "path": "src/HotChocolate/Fusion/src/Fusion.Execution/Planning/OperationPlanner.cs", "ranges": [[4117, 4118]] }
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}

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Project coverage: 54.0% (237568/439994 lines)