Android: helper stderr reaches the client error and request log unfiltered #4026

Found while working on #4021 (#4005).

When the Android snapshot helper fails, the helper's stderr goes to the client and to the request log unfiltered:

• The first stderr line becomes the failure reason, both in the client error and in the android_snapshot_helper_failed request-log entry.
• The raw details.stderr is attached to the error.

Snapshot text written to the screen can therefore reach the client and the downloadable request log if a tagless hierarchy fragment lands on stderr (for example ret" text="…" />). Remote clients can download that request log. A sensitive fill's literal value is scrubbed only where it matches exactly, so an altered form of it in a fragment is not caught.

The checked-in helper writes XML only through base64 status chunks on stdout, so this needs stderr to carry snapshot content, and no current helper path does that. It is a defense-in-depth gap, not a reproduced leak.

Required behavior. Helper stderr that reaches the client or the request log is bounded, and lines carrying snapshot content are kept out of it. The cause line that remote providers rely on (for example uiautomator unavailable) must stay visible.