Test utilities should be extracted to shared helpers

[RobertLD]

Test files duplicate utility code:

• search.test.ts has inline insertDoc/insertChunk helpers
• Multiple test files repeat createTestDb() + MockEmbeddingProvider setup
• config.test.ts repeats env var cleanup try-finally pattern
• workflow.test.ts manually recreates chunk_embeddings table

Suggested fix: Create a tests/helpers/ directory with shared fixtures, DB factories, and cleanup utilities.