Apply explicit null variable defaults when variables are omitted

[glen-84]

Summary

• Variable coercion in Core and Fusion now applies an explicit null variable default ($value: Int = null) when the request omits the variable, as Coercing Variable Values requires. Both helpers skipped a null default, so the variable was treated as not provided and an argument default such as field(arg: Int = 5) resolved to 5 instead of null. This regressed in Adds ResultDocument to HotChocolate Core #8931 and affects every release since 16.0.0.
• The Fusion gateway now sends the coerced null to the subgraph as an explicit variable value, so the result no longer depends on how the subgraph handles the forwarded = null default.
• Non-null variables are unaffected: validation already rejects a null default on them ($a: Int! = null).

Test plan

• Core: a VariableCoercionHelper unit test for an omitted variable with a null default, and an ArgumentCoercionTests execution test where echo(value: Int = 5) returns null.
• Fusion: a VariableCoercionHelper unit test, and a gateway test whose snapshot shows the subgraph receiving "value": null and the response returning null.
• All four tests fail without the fix and pass with it on net10.0.
• The neighbouring coercion test classes (Core VariableCoercionHelperTests, ArgumentCoercionTests, VariableCoercionIntegrationTests, and CodeFirstTests; Fusion VariableCoercionHelperTests and VariableCoercionTests) pass, as do the existing tests that use null variable defaults (IdAttributeTests and SlicingArgumentsTests).

Closes #10423

[Copilot]

Copilot review overview

🟢 Approval recommended

The reviewed changes include regression coverage and no unresolved blocking issues.

Review effort: Lite
Findings: None

What changed in this PR

Updates Core and Fusion variable coercion to preserve explicit null defaults for omitted nullable variables and forward them correctly.

Changes:

• Applies explicit null variable defaults.
• Adds Core, Fusion, and gateway regression tests.
• Verifies Fusion forwards coerced null values.

File	Description
src/​HotChocolate/​Fusion/​test/​Fusion.Execution.Tests/​Execution/​VariableCoercionHelperTests.cs	Tests Fusion null-default coercion.
src/​HotChocolate/​Fusion/​test/​Fusion.AspNetCore.Tests/​VariableCoercionTests.cs	Tests gateway forwarding and execution.
src/​HotChocolate/​Fusion/​test/​Fusion.AspNetCore.Tests/​__snapshots__/​VariableCoercionTests.Omitted_Variable_With_Null_Default_Overrides_Argument_Default.yaml	Captures expected forwarded null behavior.
src/​HotChocolate/​Fusion/​src/​Fusion.Execution/​Execution/​VariableCoercionHelper.cs	Applies explicit defaults in Fusion.
src/​HotChocolate/​Core/​test/​Execution.Tests/​Processing/​VariableCoercionHelperTests.cs	Tests Core null-default coercion.
src/​HotChocolate/​Core/​test/​Execution.Tests/​Integration/​Spec/​ArgumentCoercionTests.cs	Verifies null overrides argument defaults.
src/​HotChocolate/​Core/​src/​Types/​Execution/​Processing/​VariableCoercionHelper.cs	Applies explicit defaults in Core.

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[github-actions]

Patch coverage

100.0% of changed lines covered (23/23)

File	Covered	Changed	Patch %
…/src/Types/Execution/Processing/VariableCoercionHelper.cs	8	8	100.0% 🟢
…/src/Fusion.Execution/Execution/VariableCoercionHelper.cs	15	15	100.0% 🟢

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Project coverage: 58.8% (299817/509758 lines)