Initialize the packaged GStreamer runtime in the voice host (#42631)

## Why

The voice helper previously supported only handshake and shutdown, without
validating that its packaged native runtime could be initialized safely.

## What changed

- Add an `initializeRuntime` protocol exchange that loads GStreamer and the
  required plugins from physical package paths without opening audio devices.
- Restrict plugin discovery, registry access, and native library search paths,
  and keep loaded libraries alive until the helper exits.
- Give initialization a dedicated deadline and terminate the owned helper when
  initialization is cancelled or fails.
- Allow binary-only Rust targets to disable the default Bazel library target.

## Testing

- Cover helper-only packages, cancellation, and environment filtering.
- Add an ignored integration test for initialization from a relocated prepared
  runtime and rejection of duplicate initialization.

GitOrigin-RevId: 8fff68fe26e52cb2e0722dc98fb5f124eddb0d4d

codex-rs/voice-host/tests/lifecycle.rs

@@ -151,6 +151,9 @@ async fn installed_client_rejects_mixed_builds_and_missing_helper() -> Result<()
         .close()
         .await?;
     assert!(VoiceHost::connect(&package, "wrong-build").await.is_err());
+    // Helper-only installations still handshake, but cannot claim native readiness.
+    let host = VoiceHost::connect(&package, &build_commit().await?).await?;
+    assert!(host.initialize_runtime().await.is_err());
     // The same executable elsewhere in the package must not become a fallback.
     fs::rename(&helper, bin.join(source.file_name().unwrap()))?;
     assert!(