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/core/agents/api.ts

@@ -9,6 +9,15 @@ export class AgentNameCheckError extends Error {
   constructor(
     message: string,
     public readonly reason: "backend_unreachable" | "request_failed",
+    /**
+     * Raw backend `detail` string when the failure came from a backend
+     * response carrying one. `null` when no detail was provided (e.g.
+     * network-layer failure, empty response body, unparseable body) — in
+     * which case `message` is a generated fallback like "Failed to check
+     * agent name: Bad Gateway" and the UI should prefer its own localized
+     * fallback instead of surfacing the generated string.
+     */
+    public readonly detail: string | null = null,
   ) {
     super(message);
     this.name = "AgentNameCheckError";
@@ -104,9 +113,11 @@ export async function checkAgentName(
         "backend_unreachable",
       );
     }
+    const backendDetail = typeof err.detail === "string" ? err.detail : null;
     throw new AgentNameCheckError(
-      err.detail ?? `Failed to check agent name: ${res.statusText}`,
+      backendDetail ?? `Failed to check agent name: ${res.statusText}`,
       "request_failed",
+      backendDetail,
     );
   }
   return res.json() as Promise<{ available: boolean; name: string }>;

frontend/src/core/i18n/locales/en-US.ts

@@ -204,6 +204,7 @@ export const enUS: Translations = {
     nameStepNetworkError:
       "Network request failed — check your network or backend connection",
     nameStepCheckError: "Could not verify name availability — please try again",
+    nameStepCheckErrorWithDetail: "Name check failed: {detail}",
     nameStepApiDisabledError:
       "Custom agent management is not enabled on this server. Please contact your administrator.",
     nameStepBootstrapMessage:

frontend/src/core/i18n/locales/types.ts

@@ -141,6 +141,7 @@ export interface Translations {
     nameStepAlreadyExistsError: string;
     nameStepNetworkError: string;
     nameStepCheckError: string;
+    nameStepCheckErrorWithDetail: string;
     nameStepApiDisabledError: string;
     nameStepBootstrapMessage: string;
     save: string;

frontend/src/core/i18n/locales/zh-CN.ts

@@ -192,6 +192,7 @@ export const zhCN: Translations = {
     nameStepAlreadyExistsError: "已存在同名智能体",
     nameStepNetworkError: "网络请求失败，请检查网络或后端连接",
     nameStepCheckError: "无法验证名称可用性，请稍后重试",
+    nameStepCheckErrorWithDetail: "名称校验失败：{detail}",
     nameStepApiDisabledError:
       "服务器未开启自定义智能体管理功能，请联系管理员。",
     nameStepBootstrapMessage: