Result can possibly wrap a bare mysqli connection for write queries, breaking iteration

[tyrsson]

What was done to fix the analysis findings

mago analyze flagged 3 non-existent-method findings in src/Result.php:

• rewind(): $this->resource->data_seek(0) where data_seek does not exist on plain mysqli.
• loadFromMysqliResult(): $this->resource->fetch_assoc() where fetch_assoc does not exist on plain mysqli.

$this->resource is typed mysqli|mysqli_result|mysqli_stmt. Both call sites previously assumed the resource is never a bare mysqli connection object.

Added explicit instanceof guards immediately before each call, throwing a RuntimeException if the resource is not the expected type, instead of letting the call fail with an undefined method error, and instead of suppressing the finding with @mago-expect.

Rationale

Connection::execute() can pass the raw mysqli connection object itself into Driver::createResult() when a query is a non-SELECT (write) query that succeeds (true === $resultResource ? $this->resource : $resultResource). This means a Result instance can genuinely wrap a bare mysqli connection object rather than a mysqli_result or mysqli_stmt.

Calling rewind() or otherwise iterating such a Result would previously crash with an undefined method fatal error. The guards added turn that into a clear, specific RuntimeException instead.

Fixing the underlying design gap in this PR (whether Result should even wrap a bare mysqli connection for write queries, or whether iteration over such a Result should be handled differently upstream). Chose not to suppress the mago finding because doing so would make this gap invisible going forward.