Improve TimerTask

[andystaples]

Adds the following to improve TimerTask:

1. Adds support for task cancellation, enabling patterns like:

    def orchestrator(ctx: task.OrchestrationContext, _):
        approval: task.Task[bool] = ctx.wait_for_external_event('Approval')
        timeout = ctx.create_timer(timedelta(seconds=3))
        winner = yield task.when_any([approval, timeout])
        if winner == approval:
            # Explicitly cancel the timer so it does not linger
            timeout.cancel()
            return "approved"
        else:
            return "timed out"

2. Adds support for LongTimers with configurable max_timer_interval, so that backends with limits on timer duration can be used

Resolves #95
Resolves #101

[Copilot]

Pull request overview

This PR enhances the Durable Task Python SDK’s timer functionality by adding (1) task cancellation support for timers/external-event waits and (2) long-timer chunking to accommodate backends with maximum timer duration limits (resolving #95 and #101).

Changes:

• Introduces CancellableTask/TaskCanceledError and wires cancellation into timer + external event tasks.
• Adds LongTimerTask and updates orchestration execution to chunk long timers based on a maximum interval (default 3 days).
• Expands orchestration executor tests to cover long-timer chunking, cancellation after when_any, and long retry timers.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 7 comments.

File	Description
durabletask/worker.py	Implements long-timer chunk scheduling in the runtime context and executor; adds a maximum_timer_interval configuration knob.
durabletask/task.py	Adds cancellable task primitives and long-timer task implementation.
tests/durabletask/test_orchestration_executor.py	Adds unit tests for long-timer chunking/cancellation and retry behavior with long timers.

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