chore(kits): minor parity cleanups

[cabljac]

Batch of one-liners from #2974, one commit each or per kit:

• firestore-bigquery-export: logs.start() called with no argument (§4) - chore(firestore-bigquery-export): drop unused config parameter from logs.start #3039 (merged)
• firestore-vector-search: custom-endpoint content-type check dropped (§4c) - fix(firestore-vector-search): reject non-JSON responses from the custom embeddings endpoint #3042 (open)
• firestore-send-email: formatZodError invalid_string branch dropped - fix(firestore-send-email): restore the invalid_string branch in formatZodError #3040 (open)
• storage-resize-images: BLOCK_NONE accepted where the extension threw (§4); cacheControlHeader: "" set literally (§7) - fix(storage-resize-images): reject BLOCK_NONE and treat empty Cache-Control as unset #3044 (merged)
• firestore-genai-chatbot: local SafetySetting type (no enum check); PROJECT_ID error text - fix(firestore-genai-chatbot): type safety settings against the SDK enums and restore the PROJECT_ID error text #3046 (open)
• bigquery-firestore-export: dropped log helpers - chore(bigquery-firestore-export): restore dropped log helper and MAX_STALENESS doc example #3045 (open)
• firestore-bigquery-export: MAX_STALENESS doc example 8h to 4h (the original Kits parity issues #2974 ledger line mislabeled the kit; the param lives in firestore-bigquery-export)
• delete-user-data: node-fetch@2 where the extension used global fetch - chore(delete-user-data): replace node-fetch with the Node global fetch #3041 (merged)
• firestore-translate-text: dead backfill remnants (filterLanguagesFn, unused log helpers) - chore(firestore-translate-text): delete dead backfill remnants from the kit #3043 (merged)