feat(backend): a decision request accepts several resume actions

[dawidaksamski]

What

A decision request may now offer one or more resume actions, each routed on its own port. A step can have several normal outcomes, such as a review that is complete or incomplete, and none of them counts as a rejection. Until now the second outcome had to be reject, which records rejected on the run, and a run keeps the first outcome it declares.

• reject and rerun-source stay at most one each. reject is still the only action that records an outcome. A request still needs at least one resume (resume_required).
• Every routed action has its own port. A port two actions share is refused on publish and execute as duplicate_port, on the later action's port. It replaces reject_port_equals_resume_port: with several resume actions there is no one pair to compare. A blank port still answers port_empty alone.
• The decision route is unchanged. It already finds the action by name and routes on that action's port, and edits work on any resume action. New tests pin this.
• A reason stays optional on resume; reasonRequired is honoured on reject only. No client can ask a decider for a note on a resume action yet, and the flag can be added later without touching stored requests (follow-up: decision-resume-reason-required).
• The schema only loosens, so a run parked under the old rule still decides.

Docs

• Backend README: "Decision request on a node" and "Deciding" describe the new rules. A new section, "Trying several resume actions on the local stack", walks through a review with two resume actions and a reject, using only the API.
• apps/backend/decision-request.decision-log.md: a new section, "Several resume actions", items 27 to 30.
• The JSDoc on DecisionRequest and its action types says "one or more". It reaches the published types of @workflowbuilder/temporal through BaseNode.decisionRequest, but only as comment wording, so there is no changeset. pr-check.yml may warn about that.

Not in this PR

• The AI Studio decision form still offers only the first resume action. The canvas already draws one handle per routed action. Offering every action, and a Review node with several resume actions, are the next two changes.
• Settled values after a decision, and a per-action outcome.

Testing

• Backend: 507 tests in 25 files pass; tsc, eslint and prettier are clean. New tests cover:
  • several resume actions accepted beside one reject and one rerun-source;
  • a shared port refused in either order, and three actions on one port giving one issue per later action;
  • blank ports giving only port_empty;
  • a two-resume request publishing although no edge leaves its ports;
  • a decision on the second resume action routing on its port with no outcome, with and without edits.
• The route tests passed without a source change, as expected. To prove they can fail, the validator was broken on purpose so every resume decision snapped to the first resume action: all five new tests failed.
• AI Studio: 595 tests in 42 files pass, including the contract test that parses the shipped requests with the backend schema.
• Manual, the README walkthrough on the local stack:
  • Decide incomplete: the run continues on that port, the other branch is skipped, and the run ends completed with no outcome.
  • Decide complete: the other branch runs, again with no outcome.
  • Decide reject without a reason: 400 invalid_decision with reason_required. With a reason: the run ends with outcome: 'rejected'.
  • Publish a copy whose two resume actions share a port: 400 invalid_snapshot with duplicate_port at actions.1.port.