test: restructure suite — shared conftest, markers, drop dead tests, error.py coverage

[martinkersner]

Summary

Restructures the test suite per #106. Most of the original issue was already
addressed by prior PRs (#101 added granular responses-mocked keyless tests
covering validation + liquidation/open-interest/error; #96/#103/#105 set up the
keyless CI lane). This PR closes the remaining gaps:

• Shared tests/conftest.py — moves the datamaxi/telegram/naver
  fixtures, API_KEY/BASE_URL resolution, and _FLAKY_PROD_DATA_XFAIL marker
  (previously copy-pasted verbatim) into one place. tests/__init__.py added so
  from tests.conftest import ... resolves to pytest's single conftest instance.
• Markers — register integration + smoke, drop now-unused errors,
  enable --strict-markers. Live lanes are marked so the keyless CI lane
  deselects them by marker.
• CI — keyless lane now selects by marker: pytest tests/ -m "not integration"
  (was -k "not integration"). Runs keyless, no network, across the existing
  py3.10–3.14 matrix.
• Dead code removed — no-op test_invalid_exchange/test_invalid_symbol
  (except (ValueError, Exception): pass, asserts nothing) and the duplicate
  test_candle_with_from_unix (identical args to test_candle_with_from_and_to_unix).
• tests/test_error.py — direct unit coverage for error.py
  (ClientError/ServerError field storage, ParameterRequired / AtLeastOneParameter
  message formatting), which was only exercised indirectly before.

Note: tests/util.py::mock_http_response is NOT removed — the issue called it
dead, but #101 now uses it across ~12 test modules.

Test plan

• Keyless lane exactly as CI runs it, no key, no network:
  env -u DATAMAXI_API_KEY python -m pytest tests/ -m "not integration" -q
  → 113 passed, 11 skipped, 116 deselected.
• Full collection with no key: python -m pytest --collect-only -q → 240 collected, no import errors.
• black --check . and flake8 . --statistics both clean.
• Live integration/smoke lanes not run (no key; hit prod) but collect cleanly.

Closes #106