fix: SQLite3 config type handling for .env overrides

[michalsn]

Description
This PR fixes a bug where database config values coming from .env could remain strings and then be assigned to typed SQLite3 connection properties with the wrong type. That could trigger a TypeError for synchronous or cause busyTimeout to be ignored.

The fix normalizes string config values when they are assigned to typed connection properties, and also types SQLite3\Connection::$busyTimeout as ?int so it behaves consistently. This is done in BaseConnection, where the real destination type is known, rather than trying to guess types for null values in config arrays.

Fixes #10021

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