Add memory safety tests for vec0 memory leak fixes

Add comprehensive test suite that exercises code paths fixed in the memory
leak fixes from PR asg017#258:

- vec0_free: Tests create/destroy cycles with partition, auxiliary, and
  metadata columns to verify proper cleanup of column names
- vec0_init: Tests error conditions (too many partition keys, too many
  metadata columns, invalid syntax) to verify cleanup on error paths
- vec0Update_Insert: Tests batch inserts with various column types
- vec0Update_Delete_ClearMetadata: Tests DELETE operations with metadata
- vec0Filter_knn: Tests KNN queries with metadata filters
- vec0_metadata_filter_text: Tests KNN queries with text filters
- vec0_write_metadata_value: Tests inserts with metadata values

The tests include stress tests for rapid create/insert/drop cycles and
mixed operations that would surface memory leaks when run under memory
checkers like valgrind or ASAN.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: stevemurr