[Bug report] GenAI Speech Recognition: ERROR_TYPE_INVALID_REQUEST when audio begins with silence (fromPfd)

**Describe the bug**
`SpeechRecognizer` with `MODE_ADVANCED` rejects valid PCM audio streamed via `AudioSource.fromPfd()` with `ERROR_TYPE_INVALID_REQUEST` when the audio begins with ~800ms of silence. The same code path succeeds for audio where speech starts immediately. The engine appears to apply an early silence-based rejection — it errors out in ~595ms, before the speech content at ~1000ms is even reached.

There is no documentation on audio content requirements (e.g., leading silence tolerance) or supported formats for `AudioSource.fromPfd()`. The [official sample app](https://github.com/googlesamples/mlkit/blob/master/android/speech/app/src/main/java/com/google/mlkit/genai/speech/demo/SpeechRecognitionActivity.kt) only demonstrates live microphone input, not pre-recorded audio file input.

**To Reproduce**

1. Decode two MP4 videos to raw PCM (16kHz, mono, 16-bit) using `MediaExtractor` + `MediaCodec`
2. Stream the raw PCM bytes (no WAV header) through `ParcelFileDescriptor.createPipe()` — matching the official sample's pattern
3. Pass `AudioSource.fromPfd(pipe[0])` to `SpeechRecognizer.startRecognition()`
4. Video A (speech from 0ms, source 48kHz): transcribes successfully
5. Video B (speech from ~1000ms with ~800ms leading silence, source 44.1kHz): fails with `ERROR_TYPE_INVALID_REQUEST` in 595ms
```kotlin
val pipe = ParcelFileDescriptor.createPipe()

// Background thread: write raw PCM to pipe (no WAV header)
thread {
FileOutputStream(pipe[1].fileDescriptor).use { fos ->
fos.write(rawPcmBytes) // 16kHz, mono, 16-bit PCM
}
pipe[1].close()
}

// Collect recognition results
recognizer.startRecognition(
speechRecognizerRequest { audioSource = AudioSource.fromPfd(pipe[0]) }
).collect { response ->
when (response) {
is SpeechRecognizerResponse.ErrorResponse -> {
// Video B hits this immediately:
// "Speech recognition engine is closed due to internal error: ERROR_TYPE_INVALID_REQUEST"
}
// ...
}
}
```

Audio energy analysis (RMS per 200ms window) for the failing video:
```
0ms: RMS=1241 (brief decode artifact)
200ms: RMS=0 (silence)
400ms: RMS=5 (silence)
600ms: RMS=3 (silence)
800ms: RMS=367 (transition)
1000ms: RMS=2897 (speech starts here — but API already rejected at 595ms)
```

**Workaround:** Trimming leading silence with a custom energy-based VAD before piping to ML Kit resolves the issue — Video B transcribes successfully after trimming.

**Expected behavior**

The API should either:
- Wait for speech to appear in the audio stream before making an accept/reject decision, or
- Document the leading silence tolerance and audio content requirements for `fromPfd()`
Additionally, `AudioSource.fromPfd()` documentation should specify supported audio formats (raw PCM vs WAV, expected sample rate, channels, encoding).

**SDK Info:**
- SDK Name & Version: `com.google.mlkit:genai-speech-recognition:1.0.0-alpha1`
- Mode: `SpeechRecognizerOptions.Mode.MODE_ADVANCED`
**Smartphone:**
- Device: Pixel 10
- Device OS: Android 15
- AICore: Up to date, bootloader locked
**Development Environment:**
- Android Studio Ladybug
- Kotlin 2.1.0