Result Pattern for Completion Handlers & Consistent Logging

What Was Built

• Result structs for RetentionHandler (:applied/:failed), FollowUpHandler (:applied/:skipped/:failed), EventPublisher (:published/:failed)
• FetchRunner now captures and logs handler Results at warn level on failure
• Fetch Scheduler and Scrape Scheduler wrapped in rescue StandardError with warn-level logging
• Consistent log prefixes using full class names across all completion handlers
• 9 new tests covering success, failure, and skip paths for all three handlers

Commits

• 77893eb test(04-03): add Result pattern tests for completion handlers
• e03723d feat(04-03): add Result structs to completion handlers
• a2baebf fix(04-03): add error handling to fetch and scrape schedulers
• 5bd538a feat(04-03): wire Result usage in FetchRunner for handler visibility

Tasks Completed

1. Write handler Result tests (TDD red) -- 9 new tests across 3 files, all failing on missing Result constant
2. Add Result structs to completion handlers -- RetentionHandler, FollowUpHandler, EventPublisher all return typed Results
3. Standardize logging in handlers and schedulers -- full class name prefixes, scheduler rescue with warn logging
4. Wire Result usage in FetchRunner -- log_handler_result helper, backward-compatible with nil returns
5. Verify -- 1491 runs, 0 failures, 0 RuboCop offenses; 11 errors are pre-existing HostAppHarness failures

Files Modified

• lib/source_monitor/fetching/completion/retention_handler.rb -- added Result struct, return typed result, updated log prefix
• lib/source_monitor/fetching/completion/follow_up_handler.rb -- added Result struct, collect per-item errors, return typed result, updated log prefix
• lib/source_monitor/fetching/completion/event_publisher.rb -- added Result struct, wrapped dispatch in rescue, updated log prefix
• lib/source_monitor/fetching/fetch_runner.rb -- added log_handler_result, capture handler return values
• lib/source_monitor/scheduler.rb -- added rescue StandardError with warn logging
• lib/source_monitor/scraping/scheduler.rb -- added rescue StandardError with warn logging
• test/lib/source_monitor/fetching/completion/retention_handler_test.rb -- new (3 tests)
• test/lib/source_monitor/fetching/completion/event_publisher_test.rb -- new (2 tests)
• test/lib/source_monitor/fetching/completion/follow_up_handler_test.rb -- added 4 Result tests

Deviations

None.