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/translator/concerns/paramSupport.js

@@ -14,6 +14,9 @@ const STRIP_RULES = [
   { provider: "github", match: (m) => /claude/i.test(m) && !/claude.*(opus|sonnet).*4\.6/i.test(m), drop: ["thinking", "reasoning_effort"] },
   // Cloudflare Workers AI: content must be plain string, rejects OpenAI content-part array (#1926)
   { provider: "cloudflare-ai", flattenContent: true },
+  // MiMo Desktop Preview models (account-service route): content must be plain string,
+  // rejects OpenAI content-part array. Cloud models keep their parts (mimo-v2-omni is multi-modal).
+  { provider: "xiaomi-mimo", match: /preview/i, flattenContent: true },
   { provider: "volcengine-ark", match: /glm-5/i, clampToModelMaxOutput: true },
   // VolcEngine Ark caps the Kimi family at max_tokens <= 32768, but the model's
   // advertised ceiling is far higher (Kimi-K2.7-Code resolves to maxOutput 262144),