Fix invalid BSON regex flags in DatabasePresenceVerifier::getCount()

[zigzagdev]

Summary

Description

DatabasePresenceVerifier::getCount() was passing '/i' as the flags argument to MongoDB\BSON\Regex, instead of the plain PCRE flag character 'i'.
Regex flags must only contain flag characters (i, m, x, s, ...), not a delimiter-style string.
The sibling method getMultiCount() already used the correct 'i'.

This PR has no observable effect against MongoDB's current PHP driver/server: the BSON encoder silently strips invalid flag characters before a query ever reaches the server (verified by inspecting the actual command sent via CommandSubscriber).
Still, constructing an invalid Regex value is incorrect and worth fixing for correctness / forward-compatibility.

Test Plans

Added testGetCountBuildsRegexWithValidFlags() to ValidationTest.php.
Since the invalid flags can't be observed end-to-end (see above), the test asserts against the raw Regex object passed to where(), using a lightweight stub in place of the query builder/connection.

Checklist

• Add tests and ensure they pass

Test Results

vendor/bin/phpunit tests/ValidationTest.php --testdox
PHPUnit 10.5.47 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.2.32
Configuration: /var/www/laravel-mongodb/phpunit.xml

...                                                                 3 / 3 (100%)

Time: 00:00.950, Memory: 26.00 MB

Validation (MongoDB\Laravel\Tests\Validation)
 ✔ Unique
 ✔ Exists
 ✔ Get count builds regex with valid flags

OK (3 tests, 19 assertions)

[GromNaN]

The fix is correct. MongoDB\BSON\Regex expects flag characters only (i, m, x, ...) without PCRE delimiters, so replacing '/i' with 'i' is the right change, consistent with what getMultiCount() already does.

Tests: The new testGetCountBuildsRegexWithValidFlags test correctly validates the flags using a lightweight stub, and the assertion would fail with the old code (getFlags() would return '/i'). The existing testUnique and testExists tests cover the end-to-end behavior.

Security review: No issues. preg_quote() properly escapes user input, anchors ^/$ prevent partial matches, and the regex has no quantifiers that could cause ReDoS.

Performance: The invalid / flag was stripped by the BSON encoder before reaching the server (as noted in the PR description), so no query behavior change is expected.

Thanks for spotting this inconsistency and for the thorough PR description explaining the observable impact!