[Python] Convert MCP tool inputSchema to json_schema

## Description

Need to convert tool.inputSchema to json_schema format.

## Location
- File: `py/plugins/mcp/src/genkit/plugins/mcp/client/client.py`
- Line: 245
- Current code: `# TODO: json_schema conversion from tool.inputSchema`

## Acceptance Criteria
- [ ] Implement conversion from MCP inputSchema to JSON schema
- [ ] Ensure compatibility with Genkit tool definitions