fix(task-create): handle WorkflowAlreadyStartedError gracefully

[alvinkam2001]

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/create submissions idempotent while a workflow with the same task ID remains active.

• Adds configurable workflow ID conflict handling to TemporalClient.start_workflow.
• Uses USE_EXISTING for task submission while retaining ALLOW_DUPLICATE behavior for closed executions.
• Adds unit coverage for service-level policy selection, forwarding, and backward-compatible defaults.

Confidence Score: 5/5

The PR appears safe to merge, with no actionable correctness, compatibility, or security issues identified.

The active-workflow conflict behavior is scoped to task submission, the existing closed-run reuse policy remains intact, the Temporal dependency supports the new API, and tests cover policy selection and forwarding.

Important Files Changed

Filename	Overview
src/agentex/lib/core/clients/temporal/temporal_client.py	Adds a backward-compatible conflict-policy parameter and forwards it to the supported Temporal client API.
src/agentex/lib/core/temporal/services/temporal_task_service.py	Selects USE_EXISTING for duplicate active task workflow IDs without changing closed-run reuse behavior.
tests/lib/core/services/test_temporal_task_service.py	Verifies service policy selection plus explicit and default conflict-policy forwarding.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[task/create request] --> B[TemporalTaskService.submit_task]
    B --> C[Start workflow with task ID]
    C --> D{Execution with ID active?}
    D -- Yes --> E[USE_EXISTING returns existing handle]
    D -- No, prior run closed --> F[ALLOW_DUPLICATE starts new run]
    D -- No prior run --> G[Start first workflow run]
    E --> H[Return workflow ID]
    F --> H
    G --> H

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