test: establish offline HTTP adapter contract

[Mattsface]

Summary

Establishes deterministic offline characterization tests for MlbDataAdapter and MlbResult before the HTTP transport refactor in 0.8.0.

Mocked HTTP coverage moves out of tests/external_tests/. Live MLB API tests remain external-only. Known defects are locked in with strict xfail markers so a future fix surfaces as XPASS until the markers are removed.

Why

PR 1 of the 0.8.0 HTTP reliability release. Document current adapter behavior without changing production code.

What changed

• Added tests/test_mlb_dataadapter.py (requests-mock characterization + 2 strict xfails)
• Added tests/test_mlb_result.py (MlbResult constructor behavior + 2 strict xfails)
• Trimmed tests/external_tests/mlbdataadapter/test_mlbadapter.py to live API tests only
• Removed mocked test class and obsolete skipped live 500 test
• Avoid requiring result.data is payload so a future copy-on-construct fix can preserve caller-owned data

What did not change

• No production code changes (mlb_dataadapter.py, exceptions.py, __init__.py untouched)
• No sessions, timeouts, retries, new exceptions, or public API changes
• Current 404 / 500 / exception behavior preserved as documented

How tested

poetry run pytest tests/test_mlb_dataadapter.py tests/test_mlb_result.py -v
# 18 passed, 4 xfailed

poetry run pytest tests/ --ignore=tests/external_tests
# 87 passed, 4 xfailed

poetry run pytest tests/external_tests/mlbdataadapter/test_mlbadapter.py -v
# 3 passed

Risk

Low. Test-only change. Strict xfails intentionally fail the suite on XPASS after adapter-correctness fixes until markers are removed.

Intentionally left out

• Adapter correctness fixes (fix/http-adapter-correctness)
• Sessions, timeouts, retries, exception hierarchy changes