zippy: Fix various issues

[def-]

• Remove debug prints accidentally introduced in 7986bb0.
• Keep CI_MZ_SYSTEM_PARAMETER_DEFAULT across MzRestart, Mz0dtDeploy and BackupAndRestore instead of silently reverting the flags under test on the first restart. The parsing now lives in one place.
• Make --seed reproducible: pick free object names in sorted order (set iteration depends on PYTHONHASHSEED) and replace the unseeded np.random.zipf pad sizes with random.paretovariate.
• Restore the 50% chance for upstream tables without a primary key. has_pk was always True, which made all ShiftForward/ShiftBackward actions permanent no-ops, so UPDATEs never flowed through any CDC source. Debezium sources now only pick tables with a primary key since ENVELOPE DEBEZIUM requires a key.
• Let KillClusterd alternatively require TableExists so it can fire in CopyToFromS3, which creates no views.
• Trim unexecuted actions' capabilities using the instances recorded at generation time. provides() may return fresh instances and removal compares by identity, so *IsRunning capabilities were never removed.
• Join KafkaInsertParallel background threads at the end of the test and fail on their errors instead of dropping them silently. The threads also target the current mz_service now.
• Pass mz_service in CreateDebeziumSource, it otherwise routes through balancerd's potentially stale resolver.
• Re-point balancerd at the new leader after Mz0dtDeploy, its static resolver otherwise keeps routing to the stopped old generation.
• parse_timedelta: support seconds, reject unparseable input.
• Only bootstrap Polaris in scenarios that can create Iceberg sinks.
• Remove mid-test BackupAndRestore from UserTables, sink state is not recorded across a backup&restore cycle (database-issues#9589).
• Remove unused actions (the Stop actions for external services and MzStartParameterized) and the write-only State.iceberg_ fields.
• Add DropTable/DropView actions, guarded so that no depended-upon object is dropped, and wire them into UserTables and KafkaSources. User objects were never dropped before.

Test run: https://buildkite.com/materialize/nightly/builds/17089