[codex] Improve MS discovery diagnostics

[jakobtfaber-2]

Summary

• distinguish missing FIELD subtables from unreadable FIELD tables in mosaic_day.find_valid_ms()
• include the adapter exception type/message when FIELD exists but cannot be opened
• add mocked regression coverage for valid, missing, unreadable, non-directory, and filtered MS discovery cases

Root cause

Issue #22 was reproducible because staged MS paths had FIELD/ directories, but the table adapter could not open them in this environment. The swallowed exception was RuntimeError: casatools is not installed in this environment..., so the old no FIELD table warning was misleading.

Validation

• PYTHONPATH=/workspace python3 -m pytest tests/test_mosaic_day_ms_discovery.py tests/test_batch_pipeline_dry_run_quarantine.py -q
• ruff check scripts/mosaic_day.py tests/test_mosaic_day_ms_discovery.py
• ruff format --check scripts/mosaic_day.py tests/test_mosaic_day_ms_discovery.py
• Diagnose dry-run valid-MS discovery reporting zero despite staged FIELD directories #22 dry-run command exits 0 and reports unreadable FIELD table: RuntimeError: casatools is not installed...

Note: tests/test_calibration_viability.py is not present on the current origin/main base for this isolated #22 PR.

Closes #22

[Copilot]

Pull request overview

This PR improves Measurement Set (MS) discovery diagnostics used by scripts/batch_pipeline.py --dry-run via scripts/mosaic_day.py, so operators can distinguish a genuinely missing FIELD subtable from cases where FIELD exists but cannot be opened (e.g., missing CASA tooling).

Changes:

• Update mosaic_day.find_valid_ms() to separately detect missing FIELD, non-directory FIELD, and FIELD open failures (including exception type/message).
• Add regression tests covering valid MS discovery, missing FIELD, unreadable FIELD, filtered MS names, and non-directory FIELD.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
scripts/mosaic_day.py	Improves MS discovery warnings by checking FIELD existence/type and logging adapter exceptions when FIELD can’t be opened.
tests/test_mosaic_day_ms_discovery.py	Adds mocked tests to ensure the new discovery diagnostics and filtering behavior are exercised in CI without CASA.

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