Filter bots before application listeners enrich the event

[loevgaard]

Fixes #23

Stacked on #42.

Problem

The three request-based filters ran at -850, -875 and -900, i.e. after the application's own listeners at priority 0. Enrichment is normally the expensive part of the pipeline: loading the customer, the order, its lines and addresses. For bot traffic all of that work was done and then thrown away when a filter stopped propagation a moment later.

Change

PRIORITY_FILTER moves from -900 to 600, which puts the three filters between the request populators and PRIORITY_ENRICH. They only read userData->clientUserAgent, which is populated at PRIORITY_POPULATE, so they have everything they need. Their relative order is unchanged.

PopulatePixelsSubscriber moves from 700 to 500 so the filters sit between the request populators and it, which also means the pixel provider is not consulted for bot traffic.

StopPropagationIfNoPixelsHasBeenAddedSubscriber deliberately stays late at -950: an application listener may add pixels itself, so that check has to run after enrichment.

Tests

FilterBotsSubscriberTest covers the filter itself and, with a real EventDispatcher, asserts that a listener registered at PRIORITY_ENRICH is never invoked for a bot request. The pipeline order tests are updated to the new layout, and now also assert the filters still run after the request properties they filter on are populated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.76%. Comparing base (3bc77c7) to head (04febac).

Additional details and impacted files

@@                       Coverage Diff                        @@
##             fix/22-pipeline-priorities      #43      +/-   ##
================================================================
+ Coverage                         67.79%   68.76%   +0.96%     
  Complexity                          135      135              
================================================================
  Files                                30       30              
  Lines                               413      413              
================================================================
+ Hits                                280      284       +4     
+ Misses                              133      129       -4     

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