[Feature] Add local mock API for smoke tests

[ConnorQi01]

Summary

• Add a deterministic local mock API for API smoke tests.
• Cover the current smoke user, product, order, and cart endpoints without private backend dependencies.
• Document mock API startup and API_BASE_URL usage in both READMEs.
• Add AGENTS.md synchronization rules so implementation, tests, docs, commands, and integration behavior stay aligned.

Validation

• .venv\Scripts\python.exe -m pytest tests\test_mock_api_contract.py tests\test_api_base_url_config.py -q -n 0 -p no:cacheprovider --basetemp=.pytest-basetemp-final-unit
• .venv\Scripts\python.exe -m compileall tools\mock_api tests\test_mock_api_contract.py
• API_BASE_URL=http://127.0.0.1:8010 .venv\Scripts\python.exe -m pytest API_Automation\cases -v -n 0 -m smoke -p no:cacheprovider --basetemp=.pytest-basetemp-final-smoke

Smoke result: 6 passed, 16 deselected.

Closes #29