fix(task-create): handle WorkflowAlreadyStartedError gracefully - #489

Summary

Makes task/create idempotent in the Temporal ACP path. Duplicate submits for the same task ID (e.g. load-balanced agentex-agent replicas racing on the same notification) no longer surface WorkflowAlreadyStartedError, Temporal returns the existing workflow handle instead.

Change

• TemporalTaskService.submit_task now passes id_conflict_policy=WorkflowIDConflictPolicy.USE_EXISTING.
• TemporalClient.start_workflow accepts an id_conflict_policy param (defaults to UNSPECIFIED, preserving current behavior for any other caller).

Why

WorkflowIDReusePolicy.ALLOW_DUPLICATE (already set) only governs reuse after a run closes — it doesn't help when a run is currently open. Workflows are mainly just noise as the workflow still runs

Test plan

• New unit tests in tests/lib/core/services/test_temporal_task_service.py:
  • submit_task passes USE_EXISTING to the client
  • TemporalClient.start_workflow forwards id_conflict_policy when set
  • TemporalClient.start_workflow defaults to UNSPECIFIED (backwards-compat)
• Existing TemporalTaskService tests still pass
• Sanity check in staging: fire two task/create calls with the same task ID, confirm both return 200 with the same workflow ID and no error log

Greptile Summary

The PR makes Temporal-backed task creation idempotent while a workflow with the same task ID remains open.

• Adds conflict-policy forwarding to the Temporal client wrapper while preserving the existing default.
• Uses USE_EXISTING when submitting tasks.
• Adds focused tests for service configuration, forwarding, and default behavior.

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