Closes #5

Six of the seven approved changes from the issue. Point 8 (adding final to Connection) is not included - Mockery::mock(Connection::class) in tests/Pest.php requires subclassing to build the test double and final blocks that. It needs an interface extraction before it can land.

• MysqlConfig::fromUri() was defaulting enableServerSideCancellation to true, inconsistent with the constructor and fromArray() which both default to false. Corrected.
• Removed an unnecessary new SplQueue() allocation in releaseClean() that replaced the existing queue object when it was already empty.
• Removed a redundant $this->activeConnections = 0 in checkShutdownComplete(). Active connections are already zero at that point.
• Renamed the $MysqlConfig property on PoolManager to $mysqlConfig to match PSR naming.
• Removed a duplicate docblock above createNewConnection().
• Extracted the repeated queue-drain loop in healthCheck() into a single closure shared by both the resolve and reject callbacks.
• Added a unit test covering the fromUri() default correction.