feat(mcp): add operational mutation tools and unit tests for dagster_plus

[cbini]

Summary

Follow-up from #3547. Two areas identified during MCP server review that should be addressed separately.

1. Operational mutations

The server can observe everything but can only act on launches and re-executions. These mutations exist in the Dagster GraphQL schema and would fill critical ops gaps:

High priority:

• terminate_runs — cancel stuck/unwanted runs (terminateRuns mutation with SAFE_TERMINATE / MARK_AS_CANCELED_IMMEDIATELY policies)
• cancel_backfill / resume_backfill — cancelPartitionBackfill / resumePartitionBackfill mutations (complement existing list_backfills/get_backfill which already return hasCancelPermission/hasResumePermission)
• start_schedule / stop_schedule — startSchedule / stopRunningSchedule mutations
• start_sensor / stop_sensor — startSensor / stopSensor mutations
• reload_code_location — reloadRepositoryLocation mutation

Medium priority:

• list_schedules / list_sensors — discovery tools (no way to find schedule/sensor names today without knowing them)
• free_concurrency_slots — unblock stuck pipelines
• set_sensor_cursor — reset sensor tracking position

All mutation tools should use the existing confirm flag pattern (confirm=False for preview, confirm=True for execute).

2. Unit tests

The _handle_gql_errors decorator, _build_execution_params, RunSpec validation, and client-side filtering logic (e.g., get_run_logs with filter_types) are testable without hitting the API. Mock gql() and verify input/output transforms.

References

• Query sources: https://github.com/dagster-io/dagster/tree/master/js_modules/ui-core/src
• Schema: https://github.com/dagster-io/dagster/tree/master/python_modules/dagster-graphql/dagster_graphql/schema