feat(codebuddy-cn): add API key auth + credit quota tracker

Two additions on top of the merged CodeBuddy CN OAuth provider:

1. API key auth — connect with a direct API key (sent as
   Authorization: Bearer), not only via OAuth device-code flow.
   - registry: add authModes ["oauth","apikey"] + hasOAuth; combined Bearer
     auth already forwards the key, token-refresh skips key connections.
   - providers POST: accept dual-auth providers (authModes includes
     "apikey") that live under category "oauth" — previously rejected as
     "Invalid provider". Also fixes the same latent gap for xai.

2. Quota tracker — surface CodeBuddy CN credit balance on the usage
   dashboard for both OAuth and API-key connections.
   - registry: add transport.usage.url (Tencent billing endpoint) +
     features.usage/usageApikey so the connection is quota-eligible.
   - new CN-scoped handler services/usage/codebuddy-cn.js: POST the billing
     meter endpoint, unwrap data.Response.Data.Accounts[]. The payload mixes
     two credit types that must not be merged:
       * refill/base ("基础体验包") — recurring allowance; cycle resets well
         before the resource expires (CycleEndTime << DeductionEndTime).
         Reads the *Cycle* balance, resetAt = next refresh. Cadence-labelled.
       * bonus ("活动赠送包") — one-shot credits that expire at CycleEndTime.
         Reads the plain Capacity balance. Labelled "Bonus Pack N".
     One quota row per package, soonest-expiring first.
   - register handler under "codebuddy-cn" in USAGE_HANDLERS.

Frontend needs no change — USAGE_SUPPORTED_PROVIDERS/USAGE_APIKEY_PROVIDERS
and the generic parseQuotaData branch already cover this shape.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/app/api/providers/route.js

@@ -102,8 +102,12 @@ export async function POST(request) {
 
     // Validation
     const isWebCookieProvider = !!WEB_COOKIE_PROVIDERS[provider];
+    // Dual-auth providers (e.g. codebuddy-cn, xai) live under category "oauth" but also
+    // accept an API key via authModes — they aren't in APIKEY_PROVIDERS, so allow them here.
+    const supportsApiKeyMode = !!AI_PROVIDERS[provider]?.authModes?.includes("apikey");
     const isValidProvider = APIKEY_PROVIDERS[provider] ||
       FREE_TIER_PROVIDERS[provider] ||
+      supportsApiKeyMode ||
       isWebCookieProvider ||
       isOpenAICompatibleProvider(provider) ||
       isAnthropicCompatibleProvider(provider) ||