feat(xiaomi-mimo): merge MiMo Desktop support into xiaomi-mimo as dual auth

Adds the Desktop-exclusive Preview models and the Xiaomi account-session
route to the existing xiaomi-mimo provider instead of a separate
xiaomi-desktop provider, so the dashboard shows one MiMo entry rather than
three overlapping ones.

Dual auth, same pattern as kimi — API key (sk-) covers the cloud API,
Desktop/OAuth adds the account session used by the Preview models:

- registry: category oauth, authModes [oauth, apikey], oauth block, the two
  mimo-x-*-preview models, and the invite signupUrl
- executor: routes Preview models to the account-service route with a Cookie
  session, everything else keeps the sourceFormat-matched transport
- oauth: custom ECDH encrypted-callback flow (X25519 -> SHA256 -> AES-256-GCM)
  with a loopback callback proxy, plus one-click import of the local Desktop
  auth.json
- usage: weekly quota from the account session

Fixes found while merging:

- the OAuth browser flow was dead: poll-status cleared the session before the
  client could POST /exchange, so every exchange returned 400
- a Claude-format client was sent to /v1/chat/completions instead of the
  declared /anthropic/v1/messages transport, because buildUrl ignored
  runtimeTransport
- stopXiaomiMimoProxy leaked every pending session (each holding an X25519
  private key) for the process lifetime
- the OAuth exchange did not persist the Desktop passToken, so the Preview
  models could never work after a browser sign-in

Removes dead code: the local engine token minting (mimoEngine, never called
on the request path), the model-catalog and usage routes, engineToken/
engineUrl plumbing, and an unread top-level usage block.

Adds tests/unit/xiaomi-mimo-{executor,oauth-session,oauth-proxy}.test.js —
the provider previously had none.

open-sse/providers/registry/xiaomi-mimo.js

@@ -1,11 +1,19 @@
 import { CLAUDE_API_HEADERS } from "../shared.js";
 
+// Dual auth (same pattern as kimi):
+//   - API key (sk-...)      → cloud API on api.xiaomimimo.com
+//   - Desktop account/OAuth → same cloud host, plus the Desktop-exclusive Preview
+//     models served by the account-service route on mimo-server-cn.xiaomimimo.com
+//     (authorized by a Xiaomi account session cookie, not the key).
+// Endpoint is picked per model in the executor, same as opencode-go's /responses split.
 export default {
   id: "xiaomi-mimo",
   priority: 290,
   alias: "xiaomi-mimo",
   aliases: [
     "mimo",
+    "mimo-desktop",
+    "xmd",
   ],
   uiAlias: "mimo",
   display: {
@@ -16,9 +24,12 @@ export default {
     website: "https://xiaomimimo.com",
     notice: {
       apiKeyUrl: "https://platform.xiaomimimo.com/console/api-keys",
+      signupUrl: "https://mimo.xiaomimimo.com/desktop/invite/",
     },
   },
-  category: "apikey",
+  category: "oauth",
+  authModes: ["oauth", "apikey"],
+  hasOAuth: true,
   serviceKinds: ["llm", "tts"],
   transport: {
     baseUrl: "https://api.xiaomimimo.com/v1/chat/completions",
@@ -39,6 +50,11 @@ export default {
     },
   ],
   models: [
+    // Desktop-exclusive — served by the account-service route, which only accepts
+    // OpenAI format, so supportedFormats pins them to the openai transport.
+    { id: "mimo-x-pro-preview", name: "MiMo-X-Pro-Preview", upstreamModelId: "xiaomi/mimo-x-pro-preview", supportedFormats: ["openai"] },
+    { id: "mimo-x-flash-preview", name: "MiMo-X-Flash-Preview", upstreamModelId: "xiaomi/mimo-x-flash-preview", supportedFormats: ["openai"] },
+    // Cloud API models (api.xiaomimimo.com/v1)
     { id: "mimo-v2.5-pro", name: "MiMo V2.5 Pro" },
     { id: "mimo-v2.5", name: "MiMo V2.5" },
     { id: "mimo-v2-omni", name: "MiMo V2 Omni" },
@@ -51,4 +67,18 @@ export default {
     authHeader: "bearer",
     format: "xiaomi-mimo-tts",
   },
+  features: {
+    usage: true,
+    usageApikey: true,
+  },
+  // Custom OAuth — non-standard ECDH encrypted-callback flow.
+  // Handled by the Xiaomi MiMo OAuth service, not the generic PKCE pipeline.
+  oauth: {
+    custom: true,
+    authorizeUrl: "https://platform.xiaomimimo.com/authorize",
+    // The callback carries ?u=<ECDH-encrypted payload> instead of ?code=.
+    // Decryption yields { uid, sk, url }.
+    callbackParam: "u",
+    kn: "mimocode",
+  },
 };