.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