Consolidates #120559 by @JoaoMarcos44 into this PR. Both PRs landed on the same
shape for #120514 — read_file_raw must be a byte-preserving mutation boundary,
framed so the backend's merged stdout is never decoded as payload — but the
sibling one step in FRONT of it was still unfenced here.
_sample_file_bytes whitespace-joined the whole reply before base64-decoding it,
so a backend that announces something on connect had that noise decoded into the
sample: "TERM" is four base64 characters and prefixes the sample with b"LDL".
That sample is the binary-admission gate read_file_raw consults, so noise there
decides whether a file is editable at all and what a refusal reports about its
bytes. Same class as the read this PR already fixed, one call earlier.
_fenced_read() is that framing extracted once — sentinel, payload segment, and
the body's own exit status in its own trailing segment — and the three byte
transports (sample, base64, od fallback) now share it instead of repeating the
split/parse/decode triplet, with _failed_read() for the diagnostic they all
surfaced by hand.
The mock double for the sample transport moves to the fenced reply shape the
other doubles in that file already build, so it stays honest about the fence.
Co-authored-by: JoaoMarcos44 <joaomarcosdias444@gmail.com>
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