Refactor EventScraperTest to delegate to datamachine/test-handler

[chubes4]

Context

During the ability contract alignment work (data-machine#999), we identified that data-machine-events/test-event-scraper (in inc/Abilities/EventScraperTest.php) directly imports and instantiates UniversalWebScraper to call get_fetch_data() — the same thing the core datamachine/test-handler ability already does generically.

Current architecture

EventScraperTest.php
  └── new UniversalWebScraper()        ← direct import, tight coupling
      └── handler->get_fetch_data()
      └── domain-specific analysis (coverage, extraction info, venue metadata)

Proposed architecture

EventScraperTest.php
  └── wp_get_ability('datamachine/test-handler')
      └── ->execute({ handler_slug: 'universal_web_scraper', config: { source_url: $target_url } })
      └── domain-specific analysis (coverage, extraction info, venue metadata)

Why the events ability should stay

The events version is NOT just a wrapper. After fetching, it does substantial domain analysis:

1. Log capture — intercepts datamachine_log warnings from the scrape
2. Packet interpretation — parses JSON body, extracts event object
3. 3 extraction pathways — raw HTML fallback, vision flyer, structured event
4. Coverage analysis — missing time, missing venue, incomplete address, time data warnings
5. Venue metadata merging — assembles structured event_data from packet + venue_metadata
6. Domain-specific extraction_info — source_type, extraction_method, payload_type

This is events-domain knowledge that belongs in data-machine-events, not in core.

What to do

1. Replace the direct new UniversalWebScraper() + get_fetch_data() with a call to datamachine/test-handler ability
2. Keep all the domain-specific analysis code (coverage, extraction, venue merging)
3. The core ability returns packet summaries — EventScraperTest would need access to raw packet data, so check if datamachine/test-handler returns enough, or if it needs a raw_packets option

Slug reference cleanup

The consumer in extrachill-events (VenueQualificationAbilities.php line 356) still references the old slug datamachine/test-event-scraper. This was renamed to data-machine-events/test-event-scraper in PR #187 but the cross-plugin consumer wasn't updated. Fix as part of this or separately.

Refs data-machine#999