fix(migrations): report membership counts - #13598

What

MigrationReport had no rule for memberships, so the count the Appwrite source already computes was dropped on the way out.

The source counts memberships per team while building a report (reportAuth walks each team and adds listMemberships(...)->total), but with no addRule for it the value never reaches the response. A report came back showing teams with nothing behind them, and a caller sizing up a migration had no way to see how many memberships it would move.

Observed against a real 1.9.6 source — the response carried user, team, provider, topic, subscriber and message, and no membership key at all despite it being requested:

{"user":3,"team":2,"provider":1,"topic":2,"subscriber":2,"message":2,...}

Changes

• MigrationReport.php — one addRule for Resource::TYPE_MEMBERSHIP, next to the team rule it belongs with.
• tests/unit/Utopia/Response/Model/MigrationReportTest.php — new, following ExecutionTest.

Testing

• vendor/bin/phpunit tests/unit/Utopia/Response/Model/MigrationReportTest.php — passes, and fails without the rule
• composer lint — passed on both files
• phpstan --level 4 on both files — no errors

Note

This adds a field to a response model, so the API specs want regenerating before release. The Generate Specs workflow is workflow_dispatch-only and publishes to appwrite/specs, so I have left it alone rather than running it from a PR branch.