Sync Midjourney/Producer task responses with Docs task schema updates

[Copilot]

Docs updates added created_at, started_at, finished_at, and elapsed fields to task responses across the affected services. This PR aligns the touched MCPs with that contract and removes a couple of stale schema/documentation mismatches surfaced during the sync.

• Task response schema alignment

  • midjourney_get_tasks_batch and producer_get_tasks_batch now return the raw API JSON shape instead of reformatting responses into custom text.
  • This preserves newly documented task metadata fields and keeps batch task responses consistent with the underlying OpenAPI schemas.

• Midjourney batch task formatting

  • Added a dedicated format_batch_task_result() helper in Midjourney utilities.
  • Switched batch task handling to use the JSON formatter directly, matching the single-task contract style.

• Producer batch task formatting

  • Removed the hand-built summary rendering from producer_get_tasks_batch.
  • Returned the upstream batch task payload as-is so task metadata and response structure remain intact.

• OpenAI task contract wording

  • Updated openai_get_task documentation to describe started_at and elapsed instead of the older duration field name used in task responses.

• Fish usage guide cleanup

  • Removed the stale opus format mention from the Fish usage guide so the documented format list matches the current API contract.

• Regression coverage

  • Added focused tests asserting Midjourney and Producer batch task tools preserve the API response schema.

Example of the batch-task behavior change:

# before: custom human-readable summary text
result = await producer_get_tasks_batch(task_ids=["task-123"])

# after: API-shaped JSON payload, including task timing metadata
{
  "count": 1,
  "items": [
    {
      "id": "task-123",
      "created_at": 1705788000.0,
      "started_at": "2026-04-05T00:00:05.000Z",
      "finished_at": 1705788060.0,
      "elapsed": 60.0,
      "response": { "success": True, "data": [...] }
    }
  ]
}

• Fixes sync: update from Docs (9ec074c docs: sync from PlatformBackend [automated]) #474