[Fusion] Fix subgraph errors being dropped on non-null fields and null list elements

[michaelstaib]

No description provided.

[Copilot]

Copilot review overview

🟡 Changes recommended

Descendant error paths are incorrectly truncated to the null list element path.

Review effort: Balanced
Findings: 1

Open (1)

• Preserve descendant field paths in propagated GraphQL errors · New

What changed in this PR

Fixes Fusion gateway error forwarding for non-null fields and null list elements.

Changes:

• Forwards source errors when fields retain values.
• Preserves descendant errors when list elements become null.
• Adds integration tests and snapshots for both error modes.

File	Description
ValueCompletion.cs	Adds source-error forwarding during value completion.
SourceSchemaErrorTests.cs	Adds list-element error scenarios.
...Null_Element_OnError_Propagate.yaml	Snapshots propagation behavior.
...Null_Element_OnError_Null.yaml	Snapshots null-mode behavior.
...Below_Null_List_Element_OnError_Propagate.yaml	Snapshots descendant-error propagation.
...Below_Null_List_Element_OnError_Null.yaml	Snapshots descendant errors in null mode.

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[github-actions]

Patch coverage

97.4% of changed lines covered (76/78)

File	Covered	Changed	Patch %
…/src/Fusion.Execution/Execution/Results/ValueCompletion.cs	76	78	97.4% 🟡

Uncovered changed lines (JSON)

{
  "sha": "d3fbe00fc8c92e63fb72219972391aa4b4fbda13",
  "files": [
    { "path": "src/HotChocolate/Fusion/src/Fusion.Execution/Execution/Results/ValueCompletion.cs", "ranges": [[1193, 1194]] }
  ]
}

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Project coverage: 57.9% (300981/519835 lines)