feat(trigger-service): add WorkflowServiceClient Dapr adapter (TS-IMPL-013)

[toddysm]

Summary

Implements TS-IMPL-013 — the WorkflowServiceClient Dapr service-invocation
adapter the dispatcher (TS-IMPL-014) uses to drive the Workflow Service's two
inbound Internal RPCs.

Closes #643

What's included

• custos_trigger/clients/workflow.py — outbound client over the local Dapr
  sidecar (http://{host}:{port}/v1.0/invoke/{appId}/method/{method}), mirroring
  the Workflow Service's own _dapr_invoke precedent:
  • start_run(...) → POST /internal/runs:start, returns RunRef.
  • raise_external_event(run_id, step_id, ...) →
    POST /internal/runs/{runId}/steps/{stepId}:raiseEvent (202, empty body).
  • Request bodies (StartRunRequest, RaiseExternalEventRequest) match the WF
    InternalStartRunRequest / RaiseExternalEventRequest camelCase wire
    contracts; idempotencyKey is sent in the body and as an
    Idempotency-Key header when present.
  • Error taxonomy: transport failures and 408/429/5xx raise a retryable
    WorkflowClientError; permanent 4xx and undecodable 2xx bodies raise a
    non-retryable one — so the dispatcher can backoff/retry vs. dead-letter.
  • DaprEndpoint + build_invoke_url + read_dapr_endpoint (host/port from
    DAPR_HTTP_HOST / DAPR_HTTP_PORT, app id supplied by the caller).
  • NoopWorkflowServiceClient and recording FakeWorkflowServiceClient doubles.
• pyproject.toml — promotes httpx from a dev-only to a runtime dependency
  (the client uses it at runtime).
• tests/test_workflow_client.py — async coverage of URL/endpoint helpers,
  start/raise success, idempotency header on/off, retryable 5xx/429, permanent
  4xx, transport + decode errors, and both doubles.

Acceptance

• Request bodies match the WF InternalStartRunRequest / RaiseExternalEventRequest schemas.
• Transient 5xx surface a retryable error; Fake records calls; 100% adapter coverage.
• ruff / ruff format / mypy clean; pytest ≥90% (99.9% total).

[Copilot]

Pull request overview

Adds the Trigger Service’s outbound WorkflowServiceClient implementation for invoking Workflow Service internal RPCs via the local Dapr sidecar, plus test coverage and dependency updates to support runtime httpx usage.

Changes:

• Introduces DaprWorkflowServiceClient with wire models and retryable/permanent error taxonomy for start_run and raise_external_event.
• Adds unit tests covering endpoint/url helpers, success paths, error mapping, and doubles (Noop/Fake).
• Promotes httpx to a runtime dependency and marks TS-IMPL-013 complete in the trigger-service design TODOs.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/services/trigger-service/src/custos_trigger/clients/workflow.py	Adds the Dapr service-invocation adapter, wire models, error taxonomy, and doubles for Workflow Service RPCs.
src/services/trigger-service/tests/test_workflow_client.py	Adds async unit tests for URL/env helpers, success/error behavior, and client doubles.
src/services/trigger-service/src/custos_trigger/clients/init.py	Introduces the clients package and re-exports the workflow client surface.
src/services/trigger-service/pyproject.toml	Moves httpx from dev-only to runtime dependency.
design/components/trigger-service/todos.md	Marks TS-IMPL-013 as completed.

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