fix: parse malformed schedule_days string from Robot Framework

Robot Framework's Create Dictionary converts ['mon', 'tue'] into a
string. Add _parse_schedule_days to handle JSON/Python-literal parsing
before Pydantic type validation.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docs/tasks.md

@@ -1,12 +1,3 @@
-## Task 14: Invalid Schedule Time — Should reject malformed schedule_time with 422
-
-**Test Result:** FAIL — `Url: http://127.0.0.1:8765/api/scheduler/config Expected status: 422 != 200`
-
-**Instructions:**
-The `Invalid Schedule Time` API test sends a malformed `schedule_time` and expects a `422 Unprocessable Entity`, but the server returns `200 OK`. Review the scheduler config validation. The endpoint should reject malformed schedule times (e.g., invalid format, out-of-range values) and return 422.
-
----
-
 ## Task 15: Invalid Schedule Days — Should reject invalid day abbreviations with 422
 
 **Test Result:** FAIL — `Url: http://127.0.0.1:8765/api/scheduler/config Expected status: 422 != 200`