fix(prompts): match the wire shapes the prompt version routes use

[garfiec]

Summary

Editing a prompt's body never worked against a real server, and promoting a version
reported failure when it had succeeded. Both are wire-shape mismatches on the two
prompt-version routes. Neither was visible from inside the app: the client compiled,
and the Result.Error each produced was handled exactly as designed.

Closes #317. The picker's remaining stale case is an edit, and it needed both fixes
here: the edited body had to reach the server at all, and the promotion that makes it
live had to report success so the repository's revision bump ran.

Changes

core:model / core:network

• POST /api/prompts/groups/{id}/prompts nests the version under prompt in both
  directions. The request was sent flat, so the route read the text where it expected
  an object and answered HTTP 400 to every body edit. AddPromptToGroupRequest now
  nests, reusing CreatePromptData.
• The same route answers { prompt } rather than a bare prompt; AddPromptToGroupResponse
  unwraps it. Its field is non-nullable deliberately — savePrompt answers { message }
  with HTTP 200 when the write fails, so that body fails to decode and reports the failure
  rather than a success carrying nothing the caller then promotes on.
• PATCH /api/prompts/{id}/tags/production answers { message }, not the prompt.
  Decoding it as a prompt threw, turning an accepted promotion into a Result.Error.
• That route promotes req.params.promptId and never reads a body, so the client no
  longer sends one and UpdatePromptTagRequest is removed. It carried a
  productionPromptId that was silently ignored while reading as though it chose which
  version went live.

core:data

• updatePromptProductionTag drops its request parameter and returns Result<Unit>.
  The decode failure happened before the revision bump, so an accepted promotion never
  announced itself and the composer's / picker went on offering the superseded body
  until the app restarted.

feature:chat

• A body edit now promotes the version it writes. Adding a version leaves the group's
  productionId on the old one, so without the promotion the edit is stored where nothing
  reads it — the library row, the / picker and the editor all show the production body.
  publishNewVersion reports the save only when both calls land, using the id from the
  add's response. Web promotes on save the same way (alwaysMakeProd, default on).

Testing

• New PromptsApiWireShapeTest pins both routes against live response captures from a
  v0.8.7 server: the nested request, the unwrapped response, the HTTP-200 failure body,
  and the promotion's empty request. Nothing above :core:network can see a JSON key,
  which is why these survived.
• PromptEditorSaveOutcomeTest gains cases for the promotion — that it happens, that a
  failed promotion is not reported as saved, that an id-less response is not reported as
  saved, and that an unchanged body writes no version.
• Each new behavioural guard was mutation-checked: reverting the response unwrap, the
  production-tag decode, and the promotion each fails exactly the intended tests.
• :core:network, :core:data and :feature:chat unit suites, :app:assembleDebug, and
  detekt commonMain on all four modules pass.
• Device-tested on a Pixel 10 Pro Fold emulator against a local v0.8.7 server: a body edit
  saves and reaches the library, the / picker and insertion with no restart; "Set
  Production" on an older version closes the sheet, moves the tag, and the picker follows
  with no restart.

[github-actions]

Android debug APK

Artifact: switchboard-android-debug-355
Download: switchboard-android-debug-355.zip
Retention: 90 days
Commit: b3aec5b92da08190c69d8bf7eb66f45360de8afb

Download requires a GitHub login. Installs over previous debug builds without uninstalling (stable signing key).