Add edge-case tests for cross-table lookup (#136)

[turbomam]

Summary

Adds 11 tests covering edge cases in the LookupIndex and transform_spec engine introduced in #136. All tests pass on the cross-table-lookup branch — these document and lock in existing behavior.

Why these tests

Test area	What it covers	Why it matters
Duplicate keys	LIMIT 1 returns a row when multiple rows share a key	Current behavior is silent first-match without ORDER BY. Tests document this so any future change to query ordering is caught.
Empty secondary tables	Headers-only TSV registers and queries cleanly	Real-world data pipelines encounter empty tables; verifies graceful null propagation.
LookupIndex lifecycle	close() clears state, post-close ops raise, double-close is safe	Ensures the cleanup contract is stable for callers.
Engine no-joins regression	transform_spec works when class_derivation has no joins: block	The common case (no joins) must not be broken by the join machinery. No existing test covers this path through transform_spec.
Mixed derivations	Joins and non-joins class_derivation blocks coexist in one spec	Verifies that join table registration/cleanup per block doesn't interfere with subsequent non-join blocks.

Test files

• tests/test_utils/test_lookup_index_edge_cases.py — 7 tests for LookupIndex
• tests/test_transformer/test_engine_edge_cases.py — 4 tests for transform_spec

Note on duplicate-key semantics

The test_duplicate_keys_returns_a_row test intentionally does NOT assert which duplicate row is returned — it only verifies that a row comes back and that the key matches. The docstring documents that DuckDB's storage order makes this deterministic in practice, but the API doesn't guarantee it. If you'd prefer to make this a documented constraint (e.g., "first row wins") or add a warning/error for non-unique keys, I'm happy to adjust the tests accordingly.

Test plan

• All 11 new tests pass on cross-table-lookup
• All 14 existing join/lookup tests still pass
• No changes to production code

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds targeted edge-case tests to document and lock in existing behavior for the DuckDB-backed LookupIndex and the transform_spec engine introduced with cross-table lookup support.

Changes:

• Add LookupIndex edge-case tests covering duplicate keys, empty tables, and lifecycle behavior after close().
• Add transform_spec edge-case tests covering no-joins derivations, empty joined tables, and mixed (joins + non-joins) derivations in one spec.

Reviewed changes

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

File	Description
tests/test_utils/test_lookup_index_edge_cases.py	Adds 7 tests for duplicate-key semantics, empty secondary tables, and post-close() behavior of LookupIndex.
tests/test_transformer/test_engine_edge_cases.py	Adds 4 tests to prevent regressions in transform_spec for no-joins, empty-join inputs, and mixed derivation specs.

[amc-corey-cox]

Yep, this looks good to me.