fix(mcp): validate required params before call_tool

[OiPunk]

Summary

Fixes #2449 by adding a local pre-flight validation step in MCP call_tool().

When cached tool schema contains inputSchema.required, the SDK now validates required fields before sending the request to the MCP server.

What changed

• Added _validate_required_parameters() in src/agents/mcp/server.py.
• Invoked validation from _MCPServerWithClientSession.call_tool() before remote invocation.
• Added regression tests in tests/mcp/test_client_session_retries.py covering:
  • missing required params blocks remote call
  • required params present still calls remote
  • missing tool in cache skips validation
  • absent required list skips validation
  • arguments=None path validates against required list
  • non-object arguments rejected before remote call

Why this is useful

• Prevents avoidable MCP round-trips for invalid calls
• Reduces token/latency waste during tool retries
• Returns clearer local errors to the agent loop

Validation

• uv run --with ruff ruff check src/agents/mcp/server.py tests/mcp/test_client_session_retries.py
• env -u ALL_PROXY -u all_proxy -u HTTPS_PROXY -u https_proxy -u HTTP_PROXY -u http_proxy uv run --with mypy mypy src/agents/mcp/server.py tests/mcp/test_client_session_retries.py
• env -u ALL_PROXY -u all_proxy -u HTTPS_PROXY -u https_proxy -u HTTP_PROXY -u http_proxy uv run --with pytest pytest -q tests/mcp/test_client_session_retries.py
• env -u ALL_PROXY -u all_proxy -u HTTPS_PROXY -u https_proxy -u HTTP_PROXY -u http_proxy uv run --with coverage coverage run -m pytest -q tests/mcp/test_client_session_retries.py

Changed executable lines coverage for src/agents/mcp/server.py: 100% (20/20).