Log why an event was dropped

[loevgaard]

Fixes #25

Stacked on #44.

Problem

Four subscribers stopped propagation and three returned early on consent, and not one of them logged anything. "My events do not show up in Events Manager" is the first thing every integrator hits, and the only way to answer it was to step through the event dispatcher with a debugger.

Change

Each of these now logs at debug level with the event name and event id in the context:

Listener	Reason logged
FilterBotsSubscriber	request comes from a bot, with the user agent
FilterConfiguredUserAgentsSubscriber	user agent matches filters.user_agent
FilterEmptyUserAgentSubscriber	request has no user agent
StopPropagationIfNoPixelsHasBeenAddedSubscriber	no pixels are associated with the event
AddEventToTagBagSubscriber	consent not granted
AddLibraryToTagBagSubscriber	consent not granted, or no pixels available
DispatchOnCommandBusSubscriber	consent not granted

The logger is optional in every constructor and falls back to NullLogger, and each service is wired with on-invalid="null" plus <tag name="monolog.logger" channel="setono_meta_conversions_api"/>, so the channel can be turned up on its own without touching the application log. README.md shows the Monolog handler for it.

Drive-by in the same file: AddLibraryToTagBagSubscriber had on-invalid="null" on its ConsentCheckerInterface argument while the constructor requires a non-null value. That service always exists, so it never bit, but it would have been a TypeError rather than a clear container error if it ever went missing.

Tests

A SpyLogger test double records what was logged. Each filter asserts it logs exactly one debug record naming the reason, and the bot filter also asserts nothing is logged when the event passes. StopPropagationIfNoPixelsHasBeenAddedSubscriber gets its first tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.70%. Comparing base (ee77aa5) to head (919b3dd).

Additional details and impacted files

@@                       Coverage Diff                       @@
##             fix/24-non-website-events      #46      +/-   ##
===============================================================
+ Coverage                        69.85%   76.70%   +6.84%     
- Complexity                         137      139       +2     
===============================================================
  Files                               30       30              
  Lines                              418      455      +37     
===============================================================
+ Hits                               292      349      +57     
+ Misses                             126      106      -20     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.