BE006: Recommendation Candidate Retrieval Validation and Automated Tests

[shshwtsuthar]

Ticket

• BE006

Description

• Confirms BE005 PR Be005 firebase sq lite sync #27 is merged and records the current Firestore recommendation path.
• Adds retrieval coverage for missing, empty, broad-only and malformed source categories; category query construction; no matches; duplicate/original exclusion; partial records; and query failure propagation.
• Applies a test-proven boundary fix: ignore non-string source categories and normalize accepted Firestore records into the existing strict Product contract, including a document-ID barcode fallback.

Validation

• npx jest __tests__/recommendationCandidateRetrieval.test.ts __tests__/recommendationsAllergenSafety.test.ts __tests__/getProductById.test.ts --runInBand
• Passed: 3 suites, 33 tests.

Full-suite note

• npm test -- --runInBand reaches the new candidate suite successfully but has two pre-existing unrelated failures on main: missing @firebase/rules-unit-testing/Node typings in storage.rules.test.ts, and stale shoppingCartApi.test.ts error-envelope assertions. This PR does not touch those files.

Checks

• All requirements of the ticket have been implemented, or I have commented on any exclusions
• Unit tests have been added or updated for any backend changes (if applicable)
• I have reviewed the Files Changed tab and verified it only contains relevant changes
• This PR has been reviewed and approved