Changelog: 1599 fragment

changelog.d/1599-keyless-voice-key-hint.md

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+- Without `OPENAI_API_KEY`, the mic now says the key is missing and to add it in
+  Provider Settings, instead of showing the raw server error with a hint to
+  check microphone permissions. `/api/realtime/token` returns
+  `code: "API_KEY_MISSING"` with its 503 (asturksever, #1599).