feat(assert): add replaceable substring exception expectations

[ichinya]

PHPUnit requires an exactly empty exception message, replaces earlier message expectations, and accepts named exception: arguments. Preserve these contracts during migration, including argument coercion, evaluation order and captured values across assignments.

Add the public ExpectedException::withMessage(string $message, bool $exact = true) comparison mode. exact: false uses str_contains() and replaces the previous message and mode; existing calls remain exact. withMessageContaining() continues to accumulate substrings. This removes the PCRE length limit for dynamic/repeated messages while keeping independent regex constraints intact.

Rector keeps provably nonempty messages simple and removes adjacent overwritten literals when safe. Other arguments are evaluated/coerced once through a typed closure, with the comparison flag passed by name even inside spread arguments. Handler names reserve conditional and later locals across the containing function; dynamic locals from extract() stay manual. The reverse rule leaves repeated withMessage() modes unchanged when PHPUnit cannot preserve their replacement semantics.

Generated-code compatibility: the target project needs the Assert API above before running migrations that emit exact. This API is introduced by this PR and needs an Assert release before consumers can use the new bridge output. Until then, use matching Assert code from the same checkout. An older Assert now reports Unknown named parameter $exact instead of silently ignoring a positional flag. The bridge's development dependencies do not constrain a separately installed target project. The PR is marked feat because it adds public Assert API.

Validation on the final change:

PHP	Rector fixtures	Harness unit tests	Feature tests	Assert tests
8.2.34	369 passed	171 passed	3 passed	400 passed
8.3.35	369 passed	171 passed	3 passed	400 passed
8.5.11	369 passed	171 passed	3 passed	400 passed

• Runtime controls cover variable collisions, weak/strict coercion, overwritten arguments that throw, reverse comparison modes and a 100,000-character message. Each nested result is checked by test name, status and assertion count in the Feature suite.
• All 24 forward/strict controls agree with real PHPUnit 12.5.39 and 13.4.2: 17 passes, 7 required failures and no errors. The actual Rector CLI with the full PHPUNIT_TO_TESTO set and real PHPUnit reflection preserves all verdicts; auxiliary trace checks use Testo assertions. Testo records 28 assertions and PHPUnit records 56.
• Generated closure output rejects the old one-argument withMessage() signature explicitly on PHP 8.2, 8.3 and 8.5.
• Changed PHP files pass CS Fixer under PHP 8.2; git diff --check passes. Full composer cs:diff previously reported a pre-existing difference in unmodified core/Output/Terminal/Renderer/ColorMode.php on Windows.

PHP compatibility: the minimum remains PHP 8.2. Runtime platform checks and the isolated Rector dependency check pass on PHP 8.2.34. The full development dependency check on 8.2 fails because the existing locked infection/abstract-testframework-adapter 0.5.1 requires PHP ^8.3; Composer requirements and lockfiles are unchanged.

Fixes #389.