Python: Support MCP tool binding by reference in declarative YAML

[hashwnath]

Summary

Fixes #4927

AgentFactory._parse_tool() ignores self.bindings for the McpTool case, unlike FunctionTool where bindings correctly resolve the callable. This causes pre-built MCP tool dicts passed via AgentFactory(bindings={"product-mcp": mcp_tool}) to be silently ignored — the factory always constructs the tool from YAML fields, resulting in server_url: '' and a 500 error when no url is specified in the YAML.

This PR adds binding resolution for McpTool in _parse_tool(), mirroring the existing FunctionTool pattern:

1. Explicit bindings — checks tool_resource.bindings list against self.bindings dict
2. Name-based fallback — matches tool_resource.name directly against self.bindings keys

If a binding resolves, the pre-built tool dict is returned directly. If no binding matches, the existing dict-building logic runs unchanged.

Changes

• _loader.py: Added binding-by-reference resolution in the McpTool case of _parse_tool()
• test_declarative_loader.py: Added 4 tests:
  • Explicit bindings list resolves MCP tool
  • Name-based fallback resolves MCP tool
  • Regression test matching the exact scenario from Python: [Bug]: Support MCP tool binding by reference in declarative YAML #4927 (name binding with allowedTools, no url)
  • No-match falls through to existing dict-building behavior

Test plan

• All 723 existing declarative tests pass
• New tests verify both binding resolution paths and the fallback
• Ruff format and lint clean
• Regression test verifies the exact issue scenario (no server_url: '')