feat(trigger-service): add pipeline dispatcher (TS-IMPL-014)

[toddysm]

Summary

Implements TS-IMPL-014 — the Dispatcher, the tail of the linear pipeline
(Classify → Match → Dedup → Dispatch). It turns matches into Workflow Service
Internal RPCs with retry, dead-letter, dedup-commit ordering, and fan-out loop
protection.

Closes #644

What's included

• custos_trigger/pipeline/dispatch.py — Dispatcher:
  • start match → StartRun(workflowVersionId, inputs) via the
    WorkflowServiceClient; resume match →
    RaiseExternalEvent(runId, stepId, eventName, payload).
  • Retry then dead-letter — transient failures (408/429/5xx,
    transport) are retried with exponential backoff up to
    TRIGGER_DISPATCH_MAX_RETRIES; permanent failures and exhausted budgets
    dead-letter with a trigger.dispatch.failed audit event.
  • Dedup commits only after a confirmed dispatch — the call runs inside
    Deduplicator.guard, so the dedup key is rolled back on failure and the
    redelivery can re-attempt (design § Failure Modes row 1).
  • Fan-out loop guard — a dispatch whose inbound event chain depth
    exceeds the per-tenant TRIGGER_FANOUT_MAX_DEPTH limit is rejected and
    trigger.loop.detected is audited.
  • Every RPC carries the deterministic dedup key as its idempotencyKey for
    the hard idempotency guarantee.
  • AuditSink protocol + NoopAuditSink default (real OTel/audit lands in
    TS-IMPL-019); DispatchOutcome / DispatchStatus result types.
• settings.py — adds the TRIGGER_FANOUT_MAX_DEPTH knob (default 16).
• pipeline/__init__.py — exports the dispatcher surface.
• Tests — tests/test_dispatch.py covering both arms, duplicate skip,
  retry→success, retry-exhaustion→dead-letter, non-retryable→immediate
  dead-letter, dedup rollback on failure, loop rejection + boundary, and the
  Noop audit/default-sink paths (100% on dispatch.py); plus settings coverage
  for the new knob.

Notes

• inputMapping ${{ … }} placeholder resolution is performed by the receiver
  before handing the subscription to the dispatcher; the dispatcher forwards the
  resolved input_mapping as the start inputs.
• Candidate enumeration stays with the receiver/caller (the locked SPL store has
  no list surface), matching the TS-IMPL-012 matcher design.

Acceptance

• Both dispatch arms covered; retry then dead-letter on persistent failure.
• Dedup key committed only after a confirmed dispatch; loop-depth limit rejects + audits.
• ruff / ruff format / mypy clean; pytest ≥90% (99.9% total, dispatch.py 100%).

[Copilot]

Pull request overview

Implements the Trigger Service “Dispatcher” (TS-IMPL-014), the final stage of the trigger pipeline that turns matched subscriptions into Workflow Service internal RPC calls with dedup-commit ordering, bounded retries, dead-lettering, and fan-out loop protection.

Changes:

• Adds Dispatcher with start/resume dispatch arms, exponential backoff retry, dead-letter outcomes, and depth-limit loop rejection + audit sink surface.
• Introduces TRIGGER_FANOUT_MAX_DEPTH setting (default 16) and exports dispatcher symbols from custos_trigger.pipeline.
• Adds comprehensive dispatcher unit tests plus settings coverage for the new knob; marks TS-IMPL-014 complete in the trigger-service design TODOs.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/services/trigger-service/src/custos_trigger/pipeline/dispatch.py	New dispatcher implementation with retry/dead-letter/dedup-guard ordering and loop-depth guard + auditing.
src/services/trigger-service/src/custos_trigger/settings.py	Adds TRIGGER_FANOUT_MAX_DEPTH configuration wiring and default.
src/services/trigger-service/src/custos_trigger/pipeline/init.py	Exports dispatcher types/surface from the pipeline package and updates module docstring.
src/services/trigger-service/tests/test_dispatch.py	New test suite covering start/resume dispatch, retry/dead-letter behavior, dedup rollback, and loop guard.
src/services/trigger-service/tests/test_settings.py	Extends settings tests to include the new fanout depth knob.
design/components/trigger-service/todos.md	Marks TS-IMPL-014 as completed.

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