Clarify nullable behaviour

[martincostello]

Follow-up to #3976:

• Add clarifying comments and tests.
• Fix asymmetry with handling anyOf and oneOf.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.11%. Comparing base (0ca5050) to head (c88a41f).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4094      +/-   ##
==========================================
+ Coverage   95.08%   95.11%   +0.02%     
==========================================
  Files         111      111              
  Lines        4046     4070      +24     
  Branches      820      832      +12     
==========================================
+ Hits         3847     3871      +24     
  Misses        199      199              

Flag	Coverage Δ
Linux	95.11% <100.00%> (+0.02%)	⬆️
Windows	95.11% <100.00%> (+0.02%)	⬆️
macOS	95.11% <100.00%> (+0.02%)	⬆️

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[Copilot]

🟡 Human review recommended

It changes core schema nullability behavior across composed-schema shapes and spec-version serialization semantics, which warrants a final human validation pass despite the added tests.

Pull request overview

This PR is a follow-up to #3976 that clarifies and verifies nullability behavior for composed schemas, specifically addressing symmetry between nullable and non-nullable handling when schemas use anyOf, oneOf, or allOf, and ensuring serialization remains sensible across OpenAPI/Swagger versions.

Changes:

• Add non-nullable “mirror image” behaviors for composed schemas by removing null from anyOf/oneOf members (or clearing the Null flag) when a member is non-nullable.
• Add targeted unit tests for non-nullable composed-schema scenarios and for dictionary value schemas that intentionally omit type (and nullable) across OpenAPI 3.1/3.0/Swagger 2.0.
• Add clarifying inline documentation in both SchemaGenerator and AnnotationsSchemaFilter describing where nullability is represented depending on schema shape.

File summaries

File	Description
test/Swashbuckle.AspNetCore.TestSupport/Fixtures/JsonSerializerSchemaGeneratorTestTypes.cs	Adds non-nullable fixture types used to validate composed-schema nullability behavior.
test/Swashbuckle.AspNetCore.SwaggerGen.Test/SchemaGenerator/JsonSerializerSchemaGeneratorTests.cs	Adds tests covering non-nullable removal of null from anyOf/oneOf, and serialization expectations for “no type” schemas across spec versions.
test/Swashbuckle.AspNetCore.Annotations.Test/AnnotationsSchemaFilterTests.cs	Adds tests verifying SwaggerSchema(Nullable=false) removes null from compositions and refactors the test helper to support multiple declaring types.
src/Swashbuckle.AspNetCore.SwaggerGen/SchemaGenerator/SchemaGenerator.cs	Implements non-nullable “mirror” behavior by removing/clearing null within anyOf/oneOf when nullable=false, with explanatory comments.
src/Swashbuckle.AspNetCore.Annotations/AnnotationsSchemaFilter.cs	Mirrors the same non-nullable composition handling for annotation-driven nullability, plus clarifying comments.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0
• Review effort level: Lite

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