Add realtime audio device config (#12849)

## Summary
- add top-level realtime audio config for microphone and speaker
selection
- apply configured devices when starting realtime capture and playback
- keep missing-device behavior on the system default fallback path

## Validation
- just write-config-schema
- cargo test -p codex-core realtime_audio
- cargo test -p codex-tui
- just fix -p codex-core
- just fix -p codex-tui
- just fmt

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/config.schema.json

@@ -1179,6 +1179,18 @@
       },
       "type": "object"
     },
+    "RealtimeAudioToml": {
+      "additionalProperties": false,
+      "properties": {
+        "microphone": {
+          "type": "string"
+        },
+        "speaker": {
+          "type": "string"
+        }
+      },
+      "type": "object"
+    },
     "ReasoningEffort": {
       "description": "See https://platform.openai.com/docs/guides/reasoning?api-mode=responses#get-started-with-reasoning",
       "enum": [
@@ -1540,6 +1552,15 @@
       "default": null,
       "description": "Settings for app-specific controls."
     },
+    "audio": {
+      "allOf": [
+        {
+          "$ref": "#/definitions/RealtimeAudioToml"
+        }
+      ],
+      "default": null,
+      "description": "Machine-local realtime audio device preferences used by realtime voice."
+    },
     "background_terminal_max_timeout": {
       "description": "Maximum poll window for background terminal output (`write_stdin`), in milliseconds. Default: `300000` (5 minutes).",
       "format": "uint64",