ENG-1815 Add sync attempt telemetry and stale end-task handling (#1094)

* Implement end_sync_task function with JSON response and telemetry context

- Refactor end_sync_task to return a JSON object containing task status and metadata.
- Enhance error handling to include telemetry context for better debugging.
- Update related API routes and database schema to accommodate new return type.
- Document changes in sync_functions.md to reflect the new response structure.

* Refactor sync task handling to include claimed timestamp and improve error responses

- Update EndSyncTaskRpcResult to require 'ok' and 'stale' fields.
- Enhance isEndSyncTaskRpcResult function to validate response structure.
- Modify endSyncTask function to handle claimed timestamps and improve error handling.
- Update API route to parse and validate request body for task status and timestamps.
- Document changes in sync_functions.md to reflect new requirements for end_sync_task.

* Restored the worker guard

* Update end_sync_task API to allow optional startedAt parameter and improve request body validation

- Made 'startedAt' in ParsedEndSyncTaskBody optional.
- Enhanced parseEndSyncTaskBody to accept a task status string directly.
- Updated error messages for better clarity on request body requirements.
- Adjusted RPC call to conditionally include 's_started_at' based on availability.

* Remove unnecessary success assignment in createOrUpdateDiscourseEmbedding function to streamline error handling.

* add a version to the JSON output

Author: mdroidian