Fix find_source_in_db crash on empty Sources table

[kelle]

Summary

• find_source_in_db's spatial fallback search crashed with a pandas KeyError when db.query_region(...) was called against a completely empty Sources table (it can return a columnless DataFrame in that case), breaking the very first ingest_source(..., search_db=True) into a fresh database.
• An empty Sources table can have no matches by definition, so find_source_in_db now checks for that up front (db.query(db.Sources).first() is None) and returns [] immediately, before any name search, fuzzy search, SIMBAD resolution, or query_region call. This is a single early-return guard — both query_region call sites, and their existing error behavior for genuinely bad ra_col_name/dec_col_name arguments, are unchanged.

Fixes #265.

Test plan

• Added test_find_source_in_db_empty_sources_table, which builds a fresh empty database from the template schema and asserts find_source_in_db returns [] instead of raising; the SQLAlchemy engine is disposed in a finally block.
• Verified the new test fails with the pre-fix code (reproducing the reported KeyError) and passes with the fix.
• Verified test_find_source_in_db_errors's existing expectations (bad ra_col_name/dec_col_name against a non-empty table still raises KeyError/AstroDBError as before) are unaffected.
• Ran pytest tests/test_sources.py and the full suite — no new failures versus the pre-fix baseline (remaining failures are pre-existing, caused by no SIMBAD network access in this environment).
• python -m py_compile astrodb_utils/sources.py

🤖 Generated with Claude Code

https://claude.ai/code/session_016UKKrx5abQaERu4vRaZJrP

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.35%. Comparing base (dd07e99) to head (c5e664f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #266      +/-   ##
==========================================
+ Coverage   72.10%   72.35%   +0.25%     
==========================================
  Files          19       19              
  Lines        1649     1664      +15     
  Branches      211      212       +1     
==========================================
+ Hits         1189     1204      +15     
  Misses        379      379              
  Partials       81       81              

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[Copilot]

🟡 Changes recommended

There are a few concrete follow-ups (notably resource cleanup in the new test and aligning PR description with the implemented fix) that should be addressed before approval.

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Pull request overview

This PR prevents astrodb_utils.sources.find_source_in_db from crashing when called against a brand-new database whose Sources table has zero rows, and adds a regression test to cover the empty-table scenario (Fixes #265).

Changes:

• Added an early-return guard in find_source_in_db when the Sources table is empty.
• Added a regression test that creates an empty schema-only database and asserts find_source_in_db returns [] instead of raising.

File summaries

File	Description
astrodb_utils/sources.py	Adds an empty-Sources short-circuit to avoid query_region crashing on empty tables.
tests/test_sources.py	Adds a regression test that exercises find_source_in_db against a freshly created empty database.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 3
• Review effort level: Lite

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