Add runtime.stt.transcribeAudioFile for plugin STT access

Expose audio transcription through the PluginRuntime so external
plugins (e.g. marmot) can use openclaw's media-understanding provider
framework without importing unexported internal modules.

The new transcribeAudioFile() wraps runCapability({capability: "audio"})
and reads provider/model/apiKey from tools.media.audio in the config,
matching the pattern used by the Discord VC implementation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

extensions/bluebubbles/src/monitor.test.ts

@@ -120,6 +120,9 @@ function createMockRuntime(): PluginRuntime {
     tts: {
       textToSpeechTelephony: vi.fn() as unknown as PluginRuntime["tts"]["textToSpeechTelephony"],
     },
+    stt: {
+      transcribeAudioFile: vi.fn() as unknown as PluginRuntime["stt"]["transcribeAudioFile"],
+    },
     tools: {
       createMemoryGetTool: vi.fn() as unknown as PluginRuntime["tools"]["createMemoryGetTool"],
       createMemorySearchTool: