test: fix StartCmd keep-alive race, shared DB table/query collisions, and sample 503 retries - #4419

1. **Shared `tests.StartCmd` shutdown & Keep-Alive race (all 121 call sites):** 
`StartCmd`'s returned `cleanup()` previously only deleted the temporary config file without stopping the in-process Toolbox server, waiting for `:5000` to close, or flushing `http.DefaultClient` Keep-Alive connections. Sequential `Test*` functions in multi-server packages (e.g., `TestAlloyDBCreateCluster/successful_creation`) could reuse a closing Keep-Alive socket from the prior test's server and fail immediately with `EOF` (`0.00s`). `StartCmd`'s `cleanup()` now idempotently stops the server, waits for shutdown, drops idle connections, and removes the temp file.
2. **MySQL & MSSQL `uniqueID` table isolation:** 
`CleanupMySQLTables` and `CleanupMSSQLTables` previously ran at test startup with no filter, dropping every table in `test_database` out from under any concurrent CI build (`Error 1146: Table ... doesn't exist` / `list_table_stats` mismatches). Scopes both helpers to `%_<uniqueID>` (plus sweeping leaked test tables > 1 hour old) at teardown (`t.Cleanup`) matching Postgres (#2377), and filters shared-schema list assertions (`list_tables`, `list_table_stats`, `list_table_fragmentation`, `list_tables_missing_unique_indexes`) to the current run's tables.
3. **MySQL & Postgres `list_active_queries` idle subtest contention:** 
Adds per-run `testUUID` `queryTag` / `appName` filtering to `tc1` (`invoke list_active_queries when the system is idle`) in `RunMySQLListActiveQueriesTest` and `RunPostgresListActiveQueriesTest` (completing #3901).
4. **Transient Gemini `503 UNAVAILABLE` / `429` capacity spikes in sample tests:**
 Adds bounded exponential-backoff retry (up to 3 attempts, resetting `$TABLE_NAME` between retries in `.ci/sample_tests/run_tests.sh` and in Go sample `*_test.go` harnesses) while keeping user-facing `quickstart.go` sample code untouched.
5. **serverless spark test batch contention:** 
 Adds a status filter to the serverless spark test so that there isn't contention between concurrent runs