test(polylang): boot Polylang in the test bootstrap so its suite runs

[nlemoine]

Problem

Coverage reports src/Integration/Polylang/ at 0%. Every PolylangTest silently skips via markTestSkipped('Polylang is not installed.'), because function_exists('PLL') is false during the test run even though the plugin is installed.

Root cause

Polylang 3.7+ loads its API (PLL() and the pll_* functions, in src/api.php) only from Polylang::init_context(). init() reaches init_context() only when it detects a context — admin, REST, or frontend with languages defined.

Under PHPUnit there is no request context, and languages are created per test (after Refresh_Database rolls them back), so at bootstrap init() sees an empty context, returns before requiring src/api.php, and PLL() is never defined. The existing bootstrap call (new \Polylang(); ->init();) is therefore a no-op for loading the API.

This surfaced after Polylang updated past its loader refactor (composer allows ^3.4; 3.8.4 is installed).

Fix

Force a frontend context through Polylang's own pll_context filter so init_context() runs and loads the API. This completes the original intent — TestCase::setPolylangDefaultLanguage() already recreates languages per test, assuming Polylang booted once at bootstrap.

Verification

• src/Integration/Polylang/ coverage: 0% → ~79% (88/112 statements); all six classes now exercised.
• PLUGINS=polylang … --testsuite=plugin-integration: 43 passed, 0 failures, no Polylang skips — un-skipping surfaced no real bugs.
• phpcs and PHPStan clean on tests/bootstrap.php.
• The change is guarded by $isPolylang, so non-Polylang suites are unaffected.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.42%. Comparing base (bc8219b) to head (e33aaec).

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #18      +/-   ##
============================================
+ Coverage     79.86%   87.42%   +7.55%     
  Complexity      494      494              
============================================
  Files            31       31              
  Lines          1336     1336              
============================================
+ Hits           1067     1168     +101     
+ Misses          269      168     -101     

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