feat(extensions): add event-driven task scheduler

[pandasdroid]

Summary

Adds a new event-driven task scheduler extension that provides three capabilities currently missing from the controller task system:

• Delayed task execution — Schedule tasks for future execution with configurable delays via TimerDispatcher
• Automatic task chaining — Define declarative rules that automatically create downstream tasks when a source task completes via EventChainHandler
• Retry with exponential backoff — Automatically retry failed tasks with configurable backoff policy via RetryHandler

All scheduling metadata is stored in Task.extensions['event_scheduler'], making this fully non-invasive — no modifications to the core Task schema.

Architecture

extensions/event_scheduler/
├── schema/scheduler_schema.py    # ScheduleType, RetryPolicy, EventChainRule, ScheduledTaskMixin, Config
├── core/
│   ├── timer_dispatcher.py       # Delayed task execution with background polling
│   ├── event_chain.py            # Completion-triggered task chaining
│   └── retry_handler.py          # Failed task retry with exponential backoff
├── service/
│   └── event_driven_scheduler.py # Main orchestrator integrating all components
└── __init__.py                   # Public API exports

Usage Example

from openjiuwen.extensions.event_scheduler import (
    EventDrivenScheduler,
    EventSchedulerConfig,
    EventChainRule,
    ScheduledTaskMixin,
    ScheduleType,
    RetryPolicy,
)

# Configure chain rules and retry policy
config = EventSchedulerConfig(
    chain_rules=[
        EventChainRule(
            rule_id="extract-then-validate",
            source_task_type="data_extraction",
            target_task_type="data_validation",
        )
    ],
    default_retry_policy=RetryPolicy(max_retries=3, base_delay=2.0),
)

scheduler = EventDrivenScheduler(config, task_manager, event_queue)
await scheduler.start()

# Submit a delayed task
await scheduler.submit_scheduled_task(
    session_id="session-1",
    task_type="data_extraction",
    mixin=ScheduledTaskMixin(schedule_type=ScheduleType.DELAYED, delay_seconds=30.0),
)

Test Plan

• 52 unit tests covering all components (all passing)
• Schema validation tests (RetryPolicy, EventChainRule, ScheduledTaskMixin, Config)
• TimerDispatcher tests (scheduling, cancellation, limits, lifecycle)
• EventChainHandler tests (chaining, conditions, dynamic rules)
• RetryHandler tests (retry, backoff, exhaustion, fallback policy)
• EventDrivenScheduler integration tests (submission, events, lifecycle)