.Net: fix: prevent duplicate "null" in JSON Schema type arrays for nullable parameters

[roli-lpci]

Summary

• Fixes InsertNullTypeIfRequired() producing duplicate "null" entries in JSON Schema type arrays (e.g., ["string", "null", "null"]) for Nullable<T> parameters with = null defaults
• Replaces reference-equality guard (JsonArray.Contains()) with value-based .Any() check
• Adds 3 regression tests covering both trigger paths and positive insertion

Root Cause

InsertNullTypeIfRequired() in OpenAIFunction.cs uses jsonArray.Contains(NullType) to check for existing "null" entries before adding one. JsonArray.Contains() compares JsonNode objects by reference equality — JsonNode does not override Equals(). The NullType constant ("null") creates a new JsonNode on implicit conversion, so the guard always fails and "null" is always added as a duplicate.

For Nullable<T> parameters with = null defaults, AIJsonUtilities.CreateJsonSchema() correctly produces ["string", "null"]. The strict-mode sanitizer then attempts to add "null" again — the broken guard lets it through, producing ["string", "null", "null"].

Two trigger paths reach the same bug:

1. Optional parameters (IsRequired = false) → insertNullType = true
2. Schemas with "nullable": true keyword

Changes

File	Change
dotnet/src/Connectors/Connectors.OpenAI/Core/OpenAIFunction.cs	Replace jsonArray.Contains(NullType) with value-based .Any() check (follows existing pattern in NormalizeAdditionalProperties)
dotnet/src/Connectors/Connectors.OpenAI.UnitTests/Core/OpenAIFunctionTests.cs	3 new tests: duplicate prevention for optional params, duplicate prevention for nullable keyword, positive case (null inserted when absent)

Testing

• 3 new unit tests added covering:
  • ItDoesNotInsertDuplicateNullInTypeArrayForOptionalParameter — schema with pre-existing ["string", "null"] + IsRequired = false
  • ItDoesNotInsertDuplicateNullInTypeArrayForNullableKeyword — schema with "nullable": true + IsRequired = true
  • ItInsertsNullInTypeArrayWhenAbsent — schema with ["string"] only → "null" correctly added

Fixes #13527

[Copilot]

Pull request overview

This PR fixes a strict-mode JSON Schema sanitization bug in the OpenAI connector where nullable parameters could end up with duplicate "null" entries in the schema type array (e.g. ["string","null","null"]), causing schema validation failures in strict mode.

Changes:

• Fixes InsertNullTypeIfRequired() to detect an existing "null" entry by value (instead of JsonArray.Contains() reference equality) before adding "null".
• Adds regression tests to cover both trigger paths (optional parameter insertion + nullable: true keyword) and the positive insertion case.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
dotnet/src/Connectors/Connectors.OpenAI/Core/OpenAIFunction.cs	Prevents duplicate "null" insertion by switching to a value-based scan of the type array.
dotnet/src/Connectors/Connectors.OpenAI.UnitTests/Core/OpenAIFunctionTests.cs	Adds unit tests to ensure "null" is not duplicated and is inserted when missing in strict-mode sanitization.

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