feat(tasks): add dummy job executor

[anpicci]

Summary

• add DummyJobExecutorMonitorTask, a settings-controlled periodic task that:
  • is disabled by default;
  • when enabled, searches for jobs in Received, moves them to Waiting, and schedules one executor task per job;
• add DummyJobExecutorTask, a one-shot task that simulates job execution through the valid state sequence Waiting → Matched → Running → Done, while holding a per-job MutexLock;
• register and publicly export both tasks as jobs:DummyJobExecutorMonitorTask and jobs:DummyJobExecutorTask;
• add administrator documentation for the dummy-executor environment settings;
• enable the monitor explicitly at a 10-second interval in run_local.sh;
• provide the corresponding demo-deployment enablement in fix(demo): enable dummy job executor diracx-charts#286;
• add focused tests for scheduling activation, interval validation, locking, status transitions, task fan-out, and the no-job case.

Motivation

This provides a standalone dummy job-execution pipeline for exercising the DiracX task framework end to end—job discovery, status transitions, task fan-out, and per-job locking—without requiring execution through legacy DIRAC. The monitor remains opt-in so production/default deployments do not schedule it automatically.

Closes #964.

Implementation notes

• DummyJobExecutorMonitorTask uses Size.SMALL and does not define a retry policy, since an enabled monitor runs periodically.
• DummyJobExecutorTask uses Size.LARGE and ExponentialBackoff(base_delay_seconds=10, max_retries=3).
• The executor applies the Matched → Running → Done transitions in one set_job_statuses call with increasing timestamps, because a direct transition to Done would be rejected by the state machine.
• All database dependencies (JobDB, JobLoggingDB, TaskQueueDB, and JobParametersDB) and Config are injected through the task framework.
• The monitor is controlled through:
  • DIRACX_TASKS_DUMMY_JOB_EXECUTOR_ENABLED (default: false);
  • DIRACX_TASKS_DUMMY_JOB_EXECUTOR_INTERVAL_SECONDS (default: 10, required to be positive).
• run_local.sh explicitly enables the monitor at a 10-second interval. The standard demo deployment is handled by the companion charts PR, fix(demo): enable dummy job executor diracx-charts#286.
• Deliberately out of scope: JDL handling, real matching or pilots, and real job execution.

Validation

• focused dummy-executor tests: 6 passed;
• full diracx-tasks suite: 71 passed;
• all Pixi-dependent pre-commit hooks passed with --all-files;
• DIRACX and Gubbins client-generation checks passed with no retained generated-client changes;
• Basic Tests, Integration Tests, Pixi Lock, and Deployment GitHub Actions workflows all pass.

[read-the-docs-community]

Documentation build overview

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± admin/reference/env-variables/index.html
± admin/how-to/tasks/run-task-manually/index.html