fix(frontend): surface backend detail when agent name check fails (#3048)

* fix(frontend): surface backend detail when agent name check fails

The new-agent page caught AgentNameCheckError but only branched on
reason === "backend_unreachable". Everything else (notably the 422
"Invalid agent name '...'. Must match ^[A-Za-z0-9-]+$" response from
GET /api/agents/check when the user submits a name with disallowed
characters — trailing space, dot, Chinese, invisible whitespace from
copy-paste) fell through to the generic fallback "Could not verify
name availability — please try again", swallowing the detail that
already told the user exactly what to fix.

Add a request_failed branch that surfaces err.message (which
checkAgentName already populates from the backend's detail at
core/agents/api.ts). The disabled / backend_unreachable / unknown-
error paths are unchanged.

Pin the contract with unit tests covering: 200 success, fetch
rejection, 502/503/504 network errors, agents_api disabled detail,
422 validation detail carried verbatim, statusText fallback when
detail is absent, and a regression guard against misclassifying a
422 as agents_api disabled.

Closes #3041

* fix(frontend): localise the error prefix when surfacing backend detail

The previous commit surfaced the backend's raw `err.message` on the
new-agent page when the name check failed. The detail itself is
English (backend's `_validate_agent_name` text, any 5xx business
message, etc.) and dropping it bare into a zh-CN page produced a
jarring English-among-Chinese line that didn't match neighbouring
strings like "已存在同名智能体" / "无法验证名称可用性".

Add `nameStepCheckErrorWithDetail` as a templated string ("Name
check failed: {detail}" / "名称校验失败：{detail}"), mirroring the
existing `nameStepBootstrapMessage` `{name}` template pattern. The
page wraps `err.message` in it when present and falls back to the
plain `nameStepCheckError` when the detail is empty.

Rendered output (verified locally with a Console fetch mock that
returns 500 + detail):

  zh-CN: 名称校验失败：Database connection lost: SQLAlchemy connection
         pool exhausted (max 5 connections, all in use)
  en-US: Name check failed: Database connection lost: SQLAlchemy
         connection pool exhausted (max 5 connections, all in use)

The localised prefix tells the user *what operation* failed; the
raw detail tells them *why*. Translating the detail itself would
be lossy (any unbounded backend string would need a translation
table) and would break the debuggability the previous commit
delivered.

Refs #3041

* fix(frontend): distinguish backend detail from generated fallback in AgentNameCheckError

Addresses Copilot's review on #3048: the previous commits keyed off
`err.message`, but `checkAgentName` substitutes a generated fallback
string ("Failed to check agent name: ${statusText}") when the backend
sent no detail. That guaranteed `err.message` was always truthy, made
the `nameStepCheckError` fallback branch unreachable in practice, and
could surface awkward strings like "名称校验失败：Failed to check
agent name: Bad Gateway" in the UI.

Add an explicit `detail: string | null` field to AgentNameCheckError.
`checkAgentName` populates it only when the backend response actually
carried a string `detail` (defensive guard against the dict-shaped
detail that other deer-flow endpoints use for typed error codes).
The new-agent page now selects on `err.detail` instead of `err.message`
so the localised fallback wins when no real detail exists.

Also fix the prettier formatting that broke lint-frontend CI on the
previous push.

Test changes:
- The 422 carry-through test now asserts both `detail` and `message`
  hold the backend string verbatim.
- A new "falls back to statusText in message but leaves detail null"
  test pins the contract that no real detail ⇒ no UI surface leak.
- A new "treats non-string detail as null" test guards against future
  backend schema drift toward dict-shaped detail.

Refs #3041 #3048

frontend/src/app/workspace/agents/new/page.tsx

@@ -146,6 +146,27 @@ export default function NewAgentPage() {
         err.reason === "backend_unreachable"
       ) {
         setNameError(t.agents.nameStepNetworkError);
+      } else if (
+        err instanceof AgentNameCheckError &&
+        err.reason === "request_failed"
+      ) {
+        // Surface the backend-provided detail (e.g. validation error) when
+        // one is present, wrapped in a localised prefix so zh-CN users
+        // don't see a bare English string next to the surrounding Chinese
+        // UI. Falls back to the generic localised fallback when the backend
+        // sent no detail — `err.message` is unreliable for this branch
+        // because `checkAgentName` substitutes a generated fallback string
+        // ("Failed to check agent name: ${statusText}") when `detail` is
+        // missing, so testing `err.message` would always be truthy and the
+        // generated fallback would leak through.
+        setNameError(
+          err.detail
+            ? t.agents.nameStepCheckErrorWithDetail.replace(
+                "{detail}",
+                err.detail,
+              )
+            : t.agents.nameStepCheckError,
+        );
       } else {
         setNameError(t.agents.nameStepCheckError);
       }
@@ -172,6 +193,7 @@ export default function NewAgentPage() {
     t.agents.nameStepNetworkError,
     t.agents.nameStepBootstrapMessage,
     t.agents.nameStepCheckError,
+    t.agents.nameStepCheckErrorWithDetail,
     t.agents.nameStepInvalidError,
     threadId,
   ]);