fix(notifications): isolate AFD discussion fetch from forecast to fix missing update notifications

[Orinks]

Problem

AFD (Area Forecast Discussion) update notifications never fired, even though the discussion clearly updated in the UI. Alert notifications worked fine.

Root Cause

get_notification_event_data was calling _get_nws_forecast_and_discussion, which wraps the forecast and discussion fetches in a single try/except. Any transient HTTP error on the forecast endpoint (e.g. 503) caused the entire function to return (None, None, None), silently dropping discussion_issuance_time. Without an issuance time, _check_discussion_update returned None on every poll, so notifications never fired.

Fix

• weather_client_nws.py: Extracted the forecast fetch into its own try/except inside get_nws_forecast_and_discussion — a forecast failure now logs a warning and returns forecast=None but still returns the discussion. Added a new get_nws_discussion_only() that fetches grid data + AFD without touching the forecast endpoint at all (lighter weight, no blast radius from forecast failures).

• weather_client_base.py: get_notification_event_data now calls _get_nws_discussion_only instead of _get_nws_forecast_and_discussion, eliminating the unnecessary forecast fetch from the 60-second poll path and removing the silent-suppression bug entirely. Added debug logging for discussion_issuance_time and alert count at each check.

• notification_event_manager.py: Added debug log lines in _check_discussion_update for every early-return path (no issuance_time, first-run, unchanged, updated) so diagnosing notification failures is straightforward.

Tests

• New tests/test_nws_afd_notification.py (9 tests):
  • Forecast HTTP 503 → discussion + issuance_time still returned
  • Forecast + discussion both succeed
  • Grid data failure propagates (retryable)
  • get_nws_discussion_only happy path
  • get_nws_discussion_only never calls forecast endpoint
  • get_nws_discussion_only returns (None, None) on non-retryable error
  • get_notification_event_data populates discussion_issuance_time
  • get_notification_event_data calls _get_nws_discussion_only, not forecast+discussion
  • get_notification_event_data doesn't crash on (None, None) from discussion fetch
• Updated test_split_notification_timers.py to mock _get_nws_discussion_only instead of _get_nws_forecast_and_discussion in the TestGetNotificationEventData class.

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