Improve test coverage to 100% and raise Infection thresholds

[loevgaard]

Summary

Tests only — no changes to src/.

	Before	After
Line coverage	90.0% (297/330)	100% (330/330)
Method / class coverage	82% / 62%	100% / 100%
Tests / assertions	41 / 83	80 / 188
Infection MSI / covered MSI	74% / 80%	94% / 94%
Uncovered mutants	19	0

• tests/Exception/ClientExceptionTest.php (new) — ClientException was 0% covered: all three factories, exact messages incl. subcode and user-facing title/message variants, previous exception.
• tests/ValueObject/FbTest.php (new) — the abstract Fb base via Fbp: DateTimeInterface creation time, founding-date/future bounds, subdomain-index validation and immutability, defaults.
• ClientTest — setRequestFactory/setStreamFactory (one request per pixel with the pixel's own token) and HTTP-client auto-discovery when nothing is injected, using a test DiscoveryStrategy prepended to Psr18ClientDiscovery (restored in finally, no network).
• ErrorResponseTest — the three negative tests consolidated into one data provider covering 16 malformed responses (missing required fields plus a wrong type for every field).
• FbqGeneratorTest — JSON-encode failure logs an error and returns ''.
• EventTest/ContentTest/CustomTest — default event id/time, the standard-event list, invalid action_source/delivery_category rejection, full Custom payload, unsupported value rejection.
• tests/TestLogger.php — moved out of ClientTest.php so FbqGeneratorTest can share it.
• infection.json.dist — thresholds raised 61.74 → 90 (MSI) and 76.77 → 90 (covered MSI); CLAUDE.md updated to match.

The 14 remaining escaped mutants are either untestable (random_int bounds, ceil vs floor on microtime, JSON depth 512, time() + 1 upper bound) or a pcov attribution artifact (tests whose frames in a file are all unwound by an exception get no line attribution there, so Infection doesn't select them — verified by applying the Parameters.php:96 mutant by hand).

Test plan

• vendor/bin/phpunit — 80 tests, 100% line coverage
• vendor/bin/phpstan analyse (level max, incl. tests)
• vendor/bin/ecs check
• vendor/bin/composer-dependency-analyser, composer normalize --dry-run
• vendor/bin/infection — MSI 94%, passes the raised thresholds
• PHPUnit + PHPStan on PHP 8.1 with --prefer-lowest
• Five consecutive PHPUnit runs stable (time-based tests use tolerant bounds)