fix: keep empty properties in tool's InputSchema

[hhxiao]

OpenAI will report error if InputSchema of "object" type with no "properties" field existing in its JSON representation.

openai.BadRequestError: Error code: 400 - {'error': {'message': "Invalid schema for function 'getCurrentDate': In context=(), object schema missing properties.", 'type': 'invalid_request_error', 'param': 'tools[0].function.parameters', 'code': 'invalid_function_parameters'}, 'user': '{"appkey": "egai-prd-strategyoffice-020029831-workflow-1746045106141", "session_id": "b95e59c0-a570-4f82-9905-05eff7b35eba-1758281506244515510", "user": "", "prompt_truncate": "yes"}'}

Description

Fixes #<issue_number> (if applicable)

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• MCP spec compatibility implementation
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring (no functional changes)
• Performance improvement
• Tests only (no functional changes)
• Other (please describe):

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the documentation accordingly

MCP Spec Compliance

• This PR implements a feature defined in the MCP specification
• Link to relevant spec section: Link text
• Implementation follows the specification exactly

Additional Information