feat: add universal handler dry-run command (wp datamachine test)

[chubes4]

Summary

Adds wp datamachine test <handler> — a universal dry-run command for any fetch handler. Resolves #936.

• TestHandlerAbility (inc/Abilities/Handler/TestHandlerAbility.php) — the ability that resolves handler by slug, applies config defaults, calls get_fetch_data('direct', ...), and returns packet summaries
• TestCommand (inc/Cli/Commands/TestCommand.php) — the CLI surface with --list, --describe, --config, --flow, --limit, and --format flags
• Wired up in data-machine.php (require + instantiation) and Bootstrap.php (CLI registration)

CLI Interface

# List available fetch handlers
wp datamachine test --list

# Show config schema for a handler
wp datamachine test ticketmaster --describe

# Test a handler with config
wp datamachine test ticketmaster --config='{"location":"32.7,-79.9","radius":"50"}'

# Test using an existing flow's config
wp datamachine test --flow=42

# Limit + JSON output
wp datamachine test rss --config='{"feed_url":"..."}' --limit=3 --format=json

Testing Done

Tested live on events.extrachill.com:

• --list → 13 fetch handlers listed ✅
• --describe ticketmaster → config fields table ✅
• --flow=3 (Royal American web scraper) → 26 events found ✅
• ticketmaster --config='{"location":"32.7,-79.9","radius":"50"}' → 151 events ✅
• rss --config='{"feed_url":"https://pitchfork.com/feed/feed-news/rss"}' → RSS items ✅
• Invalid handler → clean error message ✅
• JSON format → full packet data ✅