feat(server): add MCP server configuration validation (#830)

* feat(server): add MCP server configuration validation

Add comprehensive validation for MCP server configurations,
inspired by Flowise's validateMCPServerConfig implementation.

MCPServerConfig checks implemented:
- Command allowlist validation (node, npx, python, docker, uvx, etc.)
- Path traversal prevention (blocks ../, absolute paths, ~/)
- Shell command injection prevention (blocks ; & | ` $ etc.)
- Dangerous environment variable blocking (PATH, LD_PRELOAD, etc.)
- URL validation for SSE/HTTP transports (scheme, credentials)
- HTTP header injection prevention (blocks newlines)

* fix the unit test error of test_chat_request

* Added the related path cases as reviewer commented

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

tests/unit/server/test_chat_request.py

@@ -163,6 +163,6 @@ async def test_load_mcp_tools_exception_handling(
     mock_stdio_client.return_value = MagicMock()
 
     with pytest.raises(HTTPException) as exc:
-        await mcp_utils.load_mcp_tools(server_type="stdio", command="foo")  # Use await
+        await mcp_utils.load_mcp_tools(server_type="stdio", command="node")  # Use await
     assert exc.value.status_code == 500
     assert "unexpected error" in exc.value.detail