Fix OpenApiSchemaService to handle implementation different from Dictionary<,> for schema.Properties

[marcominerva]

This pull request addresses a potential runtime exception when handling OpenAPI schema properties that are stored in collections sensitive to modification during enumeration (such as SortedDictionary). The core fix involves modifying the way properties are iterated and updated, and a test is added to ensure robustness.

Bug fix for property enumeration and update:

• Changed the iteration over schema.Properties in OpenApiSchemaService.cs to first create a list of keys (schema.Properties.Keys.ToList()) and then update properties by key, preventing InvalidOperationException when schema.Properties is a collection like SortedDictionary that detects modification during enumeration.

Test coverage improvements:

• Added a new test SchemaTransformer_WithSortedDictionaryProperties_DoesNotThrow in SchemaTransformerTests.cs to verify that ResolveReferenceForSchema does not throw when schema.Properties is a SortedDictionary, ensuring the fix is validated and preventing regressions.

Fixes #67086

[dotnet-policy-service]

Thanks for your PR, @marcominerva. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[Copilot]

Pull request overview

This PR fixes a runtime InvalidOperationException in OpenAPI schema reference resolution that can occur when schema.Properties is backed by an IDictionary implementation that detects modification during enumeration (e.g., SortedDictionary). It also adds a regression test to ensure schema transformers can swap in such a dictionary without breaking document generation.

Changes:

• Update ResolveReferenceForSchema to iterate a snapshot of schema.Properties keys before mutating entries.
• Add a test that replaces schema.Properties with a SortedDictionary during schema transformation and verifies OpenAPI document generation does not throw.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/OpenApi/src/Services/Schemas/OpenApiSchemaService.cs	Avoids mutating schema.Properties while enumerating it by iterating a key snapshot first.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Transformers/SchemaTransformerTests.cs	Adds regression coverage for schema.Properties backed by a SortedDictionary.