Add connector disconnect-all integration coverage

[rmorlok]

Summary

• Add OAuth2 integration coverage for connector-level disconnect-all with a bounded timeout payload and real task polling.
• Verify multiple target connector connections are deleted/unavailable while a different connector's connection remains configured and usable.
• Cover a child revocation failure path where retries are exhausted but the parent disconnect-all task still completes and forces local disconnect.
• Share workflow worker/task polling helpers with the existing single-connection disconnect integration tests.

Tests

• go test -tags integration ./oauth2 -run 'TestConnectorDisconnectAll|TestDisconnectRevocation' -count=1 (from integration_tests)
• ./scripts/preflight.sh

Closes #580