Better nullability round-tripping

[Youssef1313]

@baywet Until we finalize the discussions and decisions in #2967, I think this PR is an improvement to the current situation.

• It fixes a bug in deserialization where a null in enum array would be dropped (the change in src/Microsoft.OpenApi/Reader/JsonNodeHelper.cs fixes this bug)
• It emits "nullable": true when we detect a null type, even if we haven't serialized a type (i.e, we allow emitting a redundant nullable property)
• We detect the pattern of enum: [ null ], and deserialize it JsonSchemaType.Null. This makes round-tripping behavior better.

[Copilot]

Pull request overview

This PR improves OpenAPI 3.0 compatibility and “round-tripping” of nullability for schemas, addressing a regression where nullability could be lost when serializing and re-reading documents.

Changes:

• Preserve null elements when deserializing JSON arrays (notably enum) by mapping them to JsonNullSentinel.
• Improve V3 schema deserialization by recognizing the pattern enum: [ null ] and converting it into Type = JsonSchemaType.Null (clearing Enum to avoid retaining a compatibility representation).
• Adjust OpenAPI 3.0 schema serialization to emit "nullable": true whenever a null type is detected, even if "type" is not serialized, and update/add tests accordingly.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/Microsoft.OpenApi.Tests/Models/OpenApiSchemaV30CompatibilityTests.cs	Adds focused compatibility tests covering nullable enum round-trip and type: null compatibility behavior for OAS 3.0 serialization/deserialization.
test/Microsoft.OpenApi.Tests/Models/OpenApiSchemaTests.cs	Updates expected OAS 3.0 output to include "nullable": true alongside enum: [ null ] compatibility schemas.
src/Microsoft.OpenApi/Reader/V3/OpenApiSchemaDeserializer.cs	Converts enum: [ null ] into JsonSchemaType.Null to improve model round-tripping.
src/Microsoft.OpenApi/Reader/JsonNodeHelper.cs	Fixes array deserialization to retain null elements by using JsonNullSentinel instead of dropping them.
src/Microsoft.OpenApi/Models/OpenApiSchema.cs	Always attempts to serialize nullable for OAS 3.0 when a null type is present (even if type is omitted), and simplifies type serialization helper.