[Analysis] Connection.php: $resource nullable typing cascades due to mago flow-analysis limitation #68

Context

Part of #60.

Data

mago analyze flags incompatible-property-type, invalid-property-assignment-value, and 2x redundant-condition around $resource (declared /** @var mysqli */ protected $resource;, non-nullable per docblock, but disconnect() genuinely sets it to null). Making the property honestly nullable (native ?mysqli or docblock-only mysqli|null) was tried and reverted: it cascades into ~6 new possible-method-access-on-null findings across every other method (beginTransaction(), commit(), rollback(), getLastGeneratedValue(), etc.), because mago does not retain null-narrowing of a class property across separate statements/methods, matching the same limitation already documented for Statement.php in #61.

These findings will be captured in a mago baseline rather than fixed or suppressed inline.