fix(#20): heid-code-review fixups — persona-endpoint SessionApiFailed parity (slice-4)

Panel (Gróa+Hulda+Regin) returned zero adapter / error-map / model→role drift;
three actionable items triaged as genuine adds:

- FIX: `_persona_state_endpoint` now catches `wt.SessionApiFailed` and returns the
  `session_api_failed` envelope with the upstream status, for parity with
  `_agents_endpoint` / session-create / admin (2/3 arms flagged it; it was the lone
  sibling letting an unmatched upstream ApiError escape as a raw 500). Confirmed
  NOT a slice-4 regression — the pre-cutover persona endpoint had the same latent
  gap — but closed here since the endpoint's error surface is already being hardened
  (it gained the ConnectFailed catch this slice).
- TESTS: dual-key NEGATIVE rows — a wrong error_code at the same status defaults to
  SessionApiFailed for `define_agent` (403, 422) and `patch_agent` (422); plus the
  flat-`field` body-parse shape for `_error_field_from_body` (only the nested
  detail.field form was exercised). Closes the assertion-symmetry gap with the
  persona route's existing negative test.
- AMEND: contract slice-4 notes document the intentional client-side `":" in
  agent_id` PRE on patch/delete (a Tier-3 id is always <user>:<name>, ADR-0019).

Suite 470 green (+5).

docs/contracts/worldtree_sdk_cutover.contract.md

@@ -239,6 +239,13 @@ re-anchor its coverage-map rows.
   (respx `httpx.ConnectError` side-effect) is the RED that proves this.
 - **`agents.get(agent_id)`** (SDK `GET /agents/{id}`) is NOT wrapped — ratatoskr has no
   `get_agent` consumer; only list/persona_state/define/patch/delete are in coverage.
+- **Client-side Tier-3-id PRE on `patch_agent` / `delete_agent`.** Both assert
+  `":" in agent_id` pre-HTTP (a Tier-3 id is always `<user_id>:<agent_name>`, ADR-0019),
+  so a non-colon id fails fast with an `AssertionError` rather than reaching the SDK's
+  route-discriminated 404 → `Tier3AgentNotFound`. Intentional fail-fast on a
+  wrong-shaped id (carried over from the retired hand-rolled wrappers); documented here
+  per the heid-code-review slice-4 precision flag (the § Error map 404 rows assume a
+  well-formed Tier-3 id reaches the route).
 
 ## Out of scope