Add analyzer "Lookup Must Return Nullable Type"

[glen-84]

Summary of the changes (Less than 80 chars)

• Add analyzer "Lookup Must Return Nullable Type".

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#8072

[Copilot]

Pull request overview

Adds a new Roslyn analyzer to enforce that members annotated with Hot Chocolate’s [Lookup] attribute return nullable types, aligning schema nullability with lookup semantics (lookups can legitimately not find a result).

Changes:

• Introduce analyzer HC0113 (“Lookup Must Return Nullable Type”) for methods and properties annotated with [Lookup].
• Add a code fix provider that offers to make the return type nullable.
• Add analyzer tests and snapshot baselines; wire the analyzer into the test harness.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/HotChocolate/Core/src/Types.Analyzers/LookupReturnsNonNullableTypeAnalyzer.cs	New analyzer that reports HC0113 when [Lookup] return types are non-nullable (incl. Task/ValueTask unwrapping).
src/HotChocolate/Core/src/Types.Analyzers/LookupReturnsNonNullableTypeCodeFixProvider.cs	New code fix provider to make lookup return types nullable.
src/HotChocolate/Core/src/Types.Analyzers/Errors.cs	Registers the new diagnostic descriptor HC0113.
src/HotChocolate/Core/test/Types.Analyzers.Tests/TestHelper.cs	Adds the new analyzer to the test analyzer set.
src/HotChocolate/Core/test/Types.Analyzers.Tests/LookupReturnsNonNullableTypeAnalyzerTests.cs	New test suite covering method/property + Task/ValueTask cases.
src/HotChocolate/Core/test/Types.Analyzers.Tests/snapshots/LookupReturnsNonNullableTypeAnalyzerTests.Method_NonNullableReturn_RaisesWarning.md	Snapshot baseline for non-nullable method return warning.
src/HotChocolate/Core/test/Types.Analyzers.Tests/snapshots/LookupReturnsNonNullableTypeAnalyzerTests.Method_NullableReturn_NoWarning.md	Snapshot baseline for nullable method return no-warning.
src/HotChocolate/Core/test/Types.Analyzers.Tests/snapshots/LookupReturnsNonNullableTypeAnalyzerTests.Method_TaskNonNullableReturn_RaisesWarning.md	Snapshot baseline for Task<T> non-nullable result warning.
src/HotChocolate/Core/test/Types.Analyzers.Tests/snapshots/LookupReturnsNonNullableTypeAnalyzerTests.Method_TaskNullableReturn_NoWarning.md	Snapshot baseline for Task<T?> no-warning.
src/HotChocolate/Core/test/Types.Analyzers.Tests/snapshots/LookupReturnsNonNullableTypeAnalyzerTests.Method_ValueTaskNonNullableReturn_RaisesWarning.md	Snapshot baseline for ValueTask<T> non-nullable result warning.
src/HotChocolate/Core/test/Types.Analyzers.Tests/snapshots/LookupReturnsNonNullableTypeAnalyzerTests.Method_NoLookupAttribute_NoWarning.md	Snapshot baseline ensuring no warning without [Lookup].
src/HotChocolate/Core/test/Types.Analyzers.Tests/snapshots/LookupReturnsNonNullableTypeAnalyzerTests.Property_NonNullableReturn_RaisesWarning.md	Snapshot baseline for non-nullable property warning.
src/HotChocolate/Core/test/Types.Analyzers.Tests/snapshots/LookupReturnsNonNullableTypeAnalyzerTests.Property_NullableReturn_NoWarning.md	Snapshot baseline for nullable property no-warning.

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