Do not let a failed send break the page

[loevgaard]

Fixes #16

Stacked on #47.

Problem

DispatchOnCommandBusSubscriber dispatches at PRIORITY_SEND, inside the visitor's request. Unless the application routes SendEvent to a transport, Messenger handles it synchronously and the SDK posts to graph.facebook.com right there. Client::sendEvent() throws ClientException for any non-200 response, Messenger wraps it in HandlerFailedException, and it propagates out of EventDispatcher::dispatch() into the controller that raised the event.

One expired access token therefore returned a 500 for every page that raises an event: product pages, checkout confirmation, registration.

Change

Dispatching is wrapped in a try/catch that logs at error level with the event name, event id and the exception. Only two things can reach it:

1. The command is handled synchronously and Meta answered with an error.
2. The command is routed to a transport and the transport itself is unavailable.

Neither is reachable once the command is routed to a working transport, because dispatch() then only hands the message to the transport and the handler runs in the worker. Messenger's retry and failure handling is untouched for routed setups.

The README now presents async routing as the expected setup with the reason spelled out, rather than as an aside after the configuration reference.

Tests

Three unit tests: the command is dispatched with the right event, nothing is dispatched without consent, and a bus that throws results in one error log record and no exception escaping.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.35%. Comparing base (2a07fca) to head (865684c).

Additional details and impacted files

@@                       Coverage Diff                       @@
##             fix/26-http-client-wiring      #48      +/-   ##
===============================================================
+ Coverage                        77.00%   77.35%   +0.34%     
- Complexity                         139      140       +1     
===============================================================
  Files                               30       30              
  Lines                              461      468       +7     
===============================================================
+ Hits                               355      362       +7     
  Misses                             106      106              

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