test: Add comprehensive tests for OpenAI API builder, mutation, and streaming functionality

[alxkm]

This PR adds comprehensive test coverage for three critical components of the OpenAI API integration:

OpenAiApiBuilderTests (11 new tests)

• Builder Pattern Testing: Validates the fluent builder API with custom paths and configurations
• Immutability Verification: Ensures builders can be reused safely without side effects
• Input Validation: Tests API key validation, null handling, and invalid path detection
• Header Management: Covers custom headers, multi-value headers, and complex header scenarios
• Flexibility Testing: Validates different API key types and minimal configuration scenarios

OpenAiChatModelMutateTests (3 new tests)

• Mutation Validation: Tests invalid base URL and null API validation during mutation
• State Preservation: Ensures unchanged fields are preserved during mutation operations
• Error Handling: Validates proper exception throwing for invalid mutation parameters

OpenAiStreamFunctionCallingHelperTest (11 new tests)

• Chunk Merging: Tests null-safe merging of streaming response chunks
• Tool Call Detection: Validates detection of tool function calls in streaming responses
• Multi-Choice Handling: Tests scenarios with multiple choices in streaming chunks
• Conversion Logic: Tests chunk-to-completion conversion with various edge cases
• Robustness: Handles empty collections, null values, and malformed data gracefully

Impact

• Better error handling and edge case coverage
• Comprehensive coverage prevents future breaking changes
• Validates builder patterns, immutability, and streaming logic

[ilayaperumalg]

@alxkm Thanks for the PR! @sunyuhan1998 Thanks for the review. Rebased, squashed and merged as 0b1a52f