test(voice): isolate VOICE_BACKEND fallback test from leaked env vars

When hub/.env (operator-local) sets GEMINI_API_KEY, the 'falls back to
elevenlabs for unknown VOICE_BACKEND values' test leaks gemini-live
into the backends list. Delete the four non-elevenlabs key env vars
defensively at the start of the test, mirroring the cleanup pattern
used by the other tests in the same describe block. No behavior change.

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

Author: heavygee

hub/src/web/routes/voice.test.ts

@@ -254,6 +254,10 @@ describe('GET /api/voice/backend', () => {
 
     test('falls back to elevenlabs for unknown VOICE_BACKEND values', async () => {
         process.env.VOICE_BACKEND = 'unknown-backend'
+        delete process.env.GEMINI_API_KEY
+        delete process.env.GOOGLE_API_KEY
+        delete process.env.DASHSCOPE_API_KEY
+        delete process.env.QWEN_API_KEY
         process.env.ELEVENLABS_API_KEY = 'test-el'
         const app = createApp()
         const headers = await authHeaders()