[neo0007777]

Summary of Changes

Closes #1218

This PR addresses critical failure modes in the cache layer and visualization workers:

1. Guard empty and None repolist queries:

   • get_uncached, retrieve_from_cache, and caching_wrapper now check if not repolist and short-circuit immediately with empty results ([] or pd.DataFrame()).
   • Prevents PostgreSQL psycopg2.errors.SyntaxError: syntax error at or near ")" caused by WHERE repo_id IN () when a user clears repository selection or callbacks trigger prior to repo ID population.
   • Prevents TypeError: 'NoneType' object is not iterable when None is passed.

2. Database connection lifecycle cleanup:

   • Uses try...finally: conn.close() across cache_query_results, get_uncached, and retrieve_from_cache.
   • In psycopg2, with pg.connect(...) as conn: only manages transactions (commit/rollback) and does not close the underlying connection upon exit, contributing to database connection exhaustion (Ensure 8Knot doesnt leak connections when materialized views not present #1170).

3. Safe SQL table identifier escaping:

   • Uses psycopg2.sql.Identifier(tablename) in retrieve_from_cache rather than raw Python string .format().

4. Centralized bounded polling helper:

   • Implements wait_for_cache(func_name, repolist, timeout=60, caller="") with deadline enforcement and exponential backoff (0.5s → 1.0s → 2.0s up to 5.0s) to provide a standardized, bounded polling utility for visualization callbacks.

5. Unit test suite:

   • Adds unit test coverage in tests/test_cache_facade.py covering all edge cases (empty lists, None, connection closing, identifier escaping, timeouts, and early returns) with all 8 tests passing.

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cc @EngCaioFonseca @cdolfi @MoralCode
Would appreciate your review on this PR! It resolves unhandled database syntax crashes and Celery worker thread exhaustion when repository selections change or initial queries load.

[neo0007777]

Hi @EngCaioFonseca, @cdolfi, @MoralCode - could you please review this PR when you have a moment?

Why this is important (in short):

1. Prevents unhandled PostgreSQL IN () syntax errors and type crashes whenever repo selection is empty/cleared or still initializing.
2. Fixes unclosed psycopg2 connections across cache_facade.py that contribute to database connection exhaustion (Ensure 8Knot doesnt leak connections when materialized views not present #1170).
3. Introduces a centralized wait_for_cache helper with monotonic deadline timeouts and progressive backoff to prevent Celery callback workers from hanging in indefinite polling loops.
4. Adds full unit test coverage in tests/test_cache_facade.py (all 8 tests passing).

Thank you!