[bug] Task cancel/delete/read routes still dispatch through stale default runtime rows

[serge-ivo]

Problem

Some task routes still dispatch to the stale default runtime row instead of the live, pin-aware runner node.

The approve route documents and uses the correct behavior:

• platform/workers/api/src/routes/instances.ts:1125 says paused tasks must reach the LIVE runner.
• platform/workers/api/src/routes/instances.ts:1127 uses requireLiveRuntime(...).

But nearby routes still use the old default-row helpers:

• GET /tasks/:taskId uses requireRuntime(...) at platform/workers/api/src/routes/instances.ts:1093.
• DELETE /tasks/:taskId uses getRuntime(...) at platform/workers/api/src/routes/instances.ts:1174.
• POST /tasks/:taskId/cancel uses requireRuntime(...) at platform/workers/api/src/routes/instances.ts:1186.
• /task-events also refreshes through getRuntime(...) at platform/workers/api/src/routes/instances.ts:1208.

On multi-machine accounts, instance_runtimes can point at a disconnected/default runner while the active task lives on a node-scoped relay. Approve was fixed for this exact class of bug, but cancel/delete/read/event refresh still diverge.

Impact

Users can see a task as active, then cancel/delete it and have the request hit the wrong relay or no-op against a stale runner. The board mirror may hide a task while the real runner keeps working, or task detail/event refresh may show stale data even though the live node is connected.

Expected Fix

Route all live task operations through the same live, pin-aware path:

• Use requireLiveRuntime(...) for mutating runner task operations.
• For reads/events, either use getBoundRunnerConn/live runtime when refreshing or skip refresh and serve the mirror when no live node exists.
• Keep runner-less mirrored tickets working as they do today.

Acceptance Criteria

• Cancel/delete reaches the same runner node that approve/create uses.
• A pinned runner that is offline returns a clear offline response rather than dispatching to another/stale row.
• Tests cover two registered nodes where the default row is stale and the live node owns the task.

[serge-ivo]

Fixed in de2505f.

The root cause worth recording: the reads reached for getRuntime because it was the only non-throwing option — requireLiveRuntime throws, and a read that must fall back to the mirror can't use it. So the shape of the helper pair pushed callers onto the stale default row.

Added getLiveRuntime, the missing non-throwing half (mirroring getRuntime/requireRuntime). Now:

• POST /tasks/:id/cancel → requireLiveRuntime: reaches the live node or returns a clear offline response, instead of dispatching at a dead relay and reporting a cancel that never happened.
• GET /tasks/:id → requireLiveRuntime inside the existing try, so no live runner falls through to the mirror as before.
• DELETE and /task-events → getLiveRuntime, mirror fallback unchanged.

Runner-less mirrored tickets behave exactly as before.