Annotate SchemaRepository for nullable reference types

[KitKeen]

Motivation

SchemaRepository.TryLookupByType already carries [NotNullWhen(true)], but because the file is not #nullable enabled the out parameter is declared as the oblivious type OpenApiSchemaReference. In a nullable-enabled caller the compiler treats the out value as non-null regardless of the return, so the NotNullWhen attribute can't actually drive null-state flow — hidden null-deref bugs slip past the compiler.

Calling code currently needs a wrapper to recover the contract:

#nullable enable
public static bool TryLookupByTypeSafe(this SchemaRepository repo,
    Type type, [NotNullWhen(true)] out OpenApiSchemaReference? schema)
{
    bool ok = repo.TryLookupByType(type, out OpenApiSchemaReference? oblivious);
    schema = ok ? oblivious : null;
    return ok;
}
#nullable restore

Change

• Add #nullable enable to SchemaRepository.cs.
• Annotate the constructor parameter / DocumentName property / TryLookupByType out parameter / local TryGet outs.
• Suppress RS0037 locally — project-wide PublicAPI nullability tracking has not yet been adopted (see TODO in Directory.Build.props), so this single-file opt-in mustn't force assembly-wide rollout. PublicAPI tracker still records the surface (no nullability info), the analyzer is satisfied.

Adds three unit tests in SchemaRepositoryTests:

• TryLookupByType_KnownType_ReturnsTrueAndNonNullReference
• TryLookupByType_UnknownType_ReturnsFalseAndNullReference
• TryLookupByType_NotNullWhenAttributeIsAppliedToOutParameter (reflection guard so the contract metadata can't be removed accidentally)

Verification

dotnet test test/Swashbuckle.AspNetCore.SwaggerGen.Test/Swashbuckle.AspNetCore.SwaggerGen.Test.csproj -c Release -f net10.0
# Passed: 721 / Failed: 0

Full test suite (slnx) green: 996 tests, 0 failures across the four test assemblies that run on net10.0.

Fixes #3659.

[martincostello]

To avoid this mismatch, _reservedIds could be changed to be of type Dictionary<Type, string?>. Otherwise the value should be verifiably non-null so we don't need to use ! here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.04%. Comparing base (134e1c4) to head (4b04f95).
⚠️ Report is 44 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3958   +/-   ##
=======================================
  Coverage   95.04%   95.04%           
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  Files         111      111           
  Lines        3958     3958           
  Branches      801      801           
=======================================
  Hits         3762     3762           
  Misses        196      196           

Flag	Coverage Δ
Linux	95.04% <100.00%> (ø)
Windows	95.04% <100.00%> (ø)
macOS	95.04% <100.00%> (ø)

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