refactor: improve language normalization and date handling utilities

- Introduced a new `normalize_lang` function to standardize language codes across the application, ensuring consistent handling of user language preferences.
- Refactored date handling utilities by adding `parse_utc_iso` and `parse_utc_iso_naive` functions for better parsing of ISO 8601 date strings, enhancing timezone awareness.
- Updated various modules to utilize the new language normalization and date parsing functions, improving code clarity and maintainability.
- Enhanced error handling in date validation to raise specific `DateRangeValidationError` exceptions, providing clearer feedback on validation issues.
- Improved test coverage for date range validation and language normalization functionalities, ensuring robustness and reliability.

duty_teller/services/group_duty_pin_service.py

@@ -14,6 +14,7 @@ from duty_teller.db.repository import (
     get_all_group_duty_pin_chat_ids,
 )
 from duty_teller.i18n import t
+from duty_teller.utils.dates import parse_utc_iso
 
 
 def format_duty_message(duty, user, tz_name: str, lang: str = "en") -> str:
@@ -35,8 +36,8 @@ def format_duty_message(duty, user, tz_name: str, lang: str = "en") -> str:
     except ZoneInfoNotFoundError:
         tz = ZoneInfo("Europe/Moscow")
         tz_name = "Europe/Moscow"
-    start_dt = datetime.fromisoformat(duty.start_at.replace("Z", "+00:00"))
-    end_dt = datetime.fromisoformat(duty.end_at.replace("Z", "+00:00"))
+    start_dt = parse_utc_iso(duty.start_at)
+    end_dt = parse_utc_iso(duty.end_at)
     start_local = start_dt.astimezone(tz)
     end_local = end_dt.astimezone(tz)
     offset_sec = (