Allow configuring the cookie domain

[loevgaard]

Fixes #29

Stacked on #50.

Problem

Meta writes _fbp and _fbc on the registrable domain and encodes the level it used in the second segment of the value: fb.1. means example.com, fb.2. means www.example.com, fb.0. means a single label host.

The bundle created host-only cookies, because no domain was ever passed to Cookie::create(), while the SDK value objects default the subdomain index to 1. So on www.example.com the cookie was scoped to that host but its value claimed it was set on the registrable domain. A visitor moving between the apex, www and a checkout subdomain got different cookies, and the browser pixel could write its own domain cookie alongside the server's host-only one, giving one person two _fbp values.

Change

New options:

setono_meta_conversions_api:
    cookies:
        domain: null          # e.g. example.com
        lifetime: '+90 days'

A new Cookie\CookieDomain service answers two questions: which domain to write on, and what subdomain index goes with it. The index is the number of dots in the domain the cookie actually ends up on, which is exactly how Meta's own parameter builder computes it:

$this->sub_domain_index = substr_count($this->etld_plus_1, '.');

It is capped at 2, since the SDK asserts the value is 0, 1 or 2.

Crucially the index is applied where the value is created, in GeneratedFbpContext and QueryBasedFbcContext, not where the cookie is written. That keeps the value in the cookie and the value sent to Meta identical. A value parsed from an existing cookie keeps the index it already had.

Tests

Ten unit tests for CookieDomain covering configured domains, a leading dot, a capped deep subdomain, the request host fallback and the no-request case, plus two new tests each for GeneratedFbpContext, QueryBasedFbcContext and StoreFbpSubscriber asserting the index and the written domain line up.

[codecov]

Codecov Report

❌ Patch coverage is 89.65517% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.29%. Comparing base (9040b2c) to head (2844117).

Files with missing lines	Patch %	Lines
src/EventSubscriber/StoreFbcSubscriber.php	0.00%	3 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##             fix/28-fbp-cookie      #51      +/-   ##
=======================================================
+ Coverage                81.42%   82.29%   +0.87%     
- Complexity                 157      164       +7     
=======================================================
  Files                       33       34       +1     
  Lines                      506      531      +25     
=======================================================
+ Hits                       412      437      +25     
  Misses                      94       94              

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