feat: add new dot-path array helpers

[michalsn]

Description
This PR:

• Adds five new dot-syntax helper functions: dot_array_has(), dot_array_set(), dot_array_unset(), dot_array_only(), and dot_array_except(), each backed by a corresponding ArrayHelper static method.
• Renames ArrayHelper::dotKeyExists() to dotHas() and updates all callers, including Validation/Rules.php and Validation/StrictRules/Rules.php. This is a safe change since ArrayHelper is marked as @internal.
• Adds a fast path in ArrayHelper::convertToArray() that uses explode() for the common case (no escaped dots), avoiding PCRE engine overhead.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide