Reduce PHPStan baseline by addressing actual issues

[dereuromark]

Summary

Removes 13 of 15 stale entries from phpstan-baseline.neon by fixing the underlying issues. All non-BC; remaining 2 entries kept in baseline.

Fixes applied

• BakeSeedCommand: drop wrong inline @var annotation, cast option value; rewrite reference foreach to track removable keys explicitly so PHPStan can follow the flow.
• AbstractAdapter: tighten generateBulkInsertSql @param type to array<int, array<string, mixed>> and cast keys to string for array_map.
• AdapterFactory: declare final constructor so new static() is safe.
• MysqlAdapter: drop redundant nested isset() check inside the same branch.
• Environment: useTransactions() is declared on MigrationInterface, so the method_exists guard was dead; the test that asserted on the redundant hasTransactions call is updated. up()/down() dispatch is now guarded by method_exists for the codepath that doesn't have a change() method.
• Manager: cast strpos result and fall back to 0 in max() for the unreachable empty case.
• TableFinder: drop redundant isset checks on list keys that explode/array_reverse always populate.
• MigrationHelper: rewrite assignment as if/else so PHPStan can narrow the variable type.

Remaining baseline entries

• Index::setUnique dynamic dispatch — early continue handles unique, but PHPStan can't track it.
• SqliteAdapter null check on query result — required for test mocks where the mocked method returns null.

BC concerns deferred

• Adding up(): void / down(): void to MigrationInterface would clear the remaining method.notFound entries on Environment but is a BC break for third parties implementing the interface directly. Suggest 6.x.