feat: Add strict_mode option to function_schema and function_tool

[Jai0401]

This PR introduces a strict_mode: bool = True option to @function_tool, allowing optional parameters when set to False. This change enables more flexibility while maintaining strict JSON schema validation by default.

resolves #43

Changes:

• Added strict_mode parameter to @function_tool and passed it to function_schema and FunctionTool.
• Updated function_schema.py to respect strict_mode and allow optional parameters when set to False.
• Added unit tests to verify optional parameters work correctly, including multiple optional params with different types.

Tests:

• Verified function calls with missing optional parameters behave as expected.
• Added async tests to validate behavior under different configurations.

[Jai0401]

Hey @codefromthecrypt, I used your suggested edge cases (e.g., multiple optional parameters of different types) as inspiration for the test cases in this PR. Thanks for the great examples! Let me know if you have any additional suggestions or feedback.

[Jai0401]

Hey @rm-openai, I've implemented the strict_mode option for @function_tool() and added test cases inspired by @codefromthecrypt's suggestions. Could you review the PR when you get a chance? Let me know if any changes are needed. Thanks!

[codefromthecrypt]

tests look easier than I thought. you found a nice place to put them!