Turn of successful githab CI notifications for Discord

[aryan-neogi]

Summary

Closes #152

This PR adds support for filtering Discord notifications based on workflow success status.

Changes

fenn/notification/services/discord.py

• Added notify_on_success parameter (default: False)
• Added should_notify() override to filter success notifications

fenn/notification/service.py

• Added base should_notify() method for notification services

fenn/notification/notifier.py

• Updated notify() to accept an is_success parameter
• Added check to call should_notify() before sending notifications

Tests

tests/unit/notification/test_discord_service.py

Added tests covering notification filtering behavior:

• Sends notifications on failure
• Skips success notifications when notify_on_success=False
• Sends success notifications when notify_on_success=True
• Verifies should_notify() logic

Result

Discord notifications can now be configured to only send on failures, reducing notification noise while preserving existing behavior through configuration.