test: Enhance test coverage for Spring AI Chat Client components

[alxkm]

Summary
Adds comprehensive test coverage for ChatClientResponseEntity, ChatClientResponse, and DefaultChatClientBuilder classes to improve error handling validation and edge case coverage.

Changes

ChatClientResponseEntityTests: Added tests for null responses, primitive types, array parsing, and malformed JSON handling
ChatClientResponseTests: Enhanced context mutation, immutability, and builder pattern edge cases
DefaultChatClientBuilderTests: Added resource loading with different encodings, large file handling, and clone behavior validation

Key Improvements

• Error handling for null/empty responses and invalid JSON
• Type safety validation for primitives and collections
• Resource loading edge cases and memory efficiency
• Builder state isolation and immutability guarantees
• Complex object preservation in contexts

Test Coverage Added

• new tests for response entity parsing
• new tests for response wrapper functionality
• new tests for builder pattern validation

Backward Compatibility

• All existing tests pass
• No API changes
• No breaking changes