test: add unit tests for utility modules (response, config, network)

[Tyrrnien81]

Description

The ros_mcp/utils/ package contains core utility functions used across all tools, but currently has no unit test coverage. The existing tests/installation/ directory only verifies package installation (Docker-based), not functional correctness.

Scope

Unit tests for the following modules:

utils/response.py

• _check_response() — validates rosbridge response structure
• _safe_get_values() — safely extracts nested values
• _extract_error() — extracts error messages from responses

utils/config_utils.py

• load_robot_config() — loads and validates robot YAML specs
• get_verified_robot_spec_util() — parses verified robot specification
• get_verified_robots_list_util() — lists available robot specifications

utils/network_utils.py

• _resolve_dns() — DNS resolution with IP detection
• ping_ip_and_port() — network connectivity diagnostics (mocked)

Proposed Structure

tests/
├── installation/          # existing
└── unit/                  # new
    ├── __init__.py
    ├── conftest.py        # shared fixtures
    ├── test_response.py
    ├── test_config_utils.py
    └── test_network_utils.py

Notes

• All network operations are mocked (no real network calls in unit tests)
• Tests use pytest fixtures for common test data
• File I/O tests use tmp_path and monkeypatch for isolation
• Compatible with existing pytest configuration in pyproject.toml
• 47 test cases covering all code branches including edge cases
• Does not overlap with stex2005's integration test PRs (refactor(detection): ROS version detection with RosVersion enum, add tool tests (step0) #276-refactor(topics): migrate to rosapi_service()/rosapi_type(), add tool tests (step3) #281) which target tests/integration/