fix(conversion): isolate malformed groups and trust converter output (fixes #112)

[jakobtfaber]

Summary

Fixes both conversion follow-ups reported in #112:

• moves timestamp extraction and ISOT parsing inside the per-group exception boundary, so one malformed group cannot abort later conversions
• uses the authoritative converted and skipped group IDs returned by convert_subband_groups_to_ms instead of reconstructing output names with a selector timestamp glob
• only reuses skipped MS paths when skip_existing=True and the returned path exists
• consolidates timestamp-string coverage into the existing conversion behavior tests

Root cause

convert_groups parsed the two-minute conversion window before its per-group try, so malformed timestamps escaped the isolation contract. After conversion it searched for {selector_timestamp}*.ms, but the core converter derives its output group ID from a separate database query; small timestamp differences could therefore create an MS that the caller silently failed to return.

Validation

• PYTHONPATH=/data/dsa110-continuum /opt/miniforge/envs/casa6/bin/python -m pytest tests/test_calibrator_ms_generator.py -q — 26 passed
• /opt/miniforge/envs/casa6/bin/python -m ruff check dsa110_continuum/conversion/calibrator_ms_generator.py tests/test_calibrator_ms_generator.py — passed
• python -m py_compile dsa110_continuum/conversion/calibrator_ms_generator.py — passed
• make test-cloud import-migration check passed, but the local run was interrupted after check_contimg_mentions.py remained blocked in an H17 filesystem request during repository-wide traversal; GitHub CI should run the same gate on the clean checkout

Closes #112.