fix(connection): tolerate non-UTF-8 command output when decoding

[plusky]

What

Connection.run() accumulated raw bytes from the channel and returned
them via stdout.decode()/stderr.decode() with the default strict
UTF-8 error mode. Any non-UTF-8 byte in a remote command's output
(locale-encoded package descriptions, binary noise) raised
UnicodeDecodeError out of run(), which is neither CommandTimeout nor
the expected result tuple, so it escaped unhandled and killed the
host worker mid-run. The streaming debug path already decoded with a
tolerant mode, making the strict final decode an oversight.

The asyncssh backend had the same defect one layer down: asyncssh
decodes session output with encoding='utf-8' and errors='strict' by
default, and its channel wraps the UnicodeDecodeError in a
ProtocolError that tears down the whole connection on the first bad
byte.

Decode the paramiko backend's final output with errors="replace" and
pass errors="replace" through AsyncConnection.run() to asyncssh, so
undecodable bytes come back as U+FFFD on both backends instead of
raising. The paramiko regression test feeds b"ok\xff\xfe"-style
output through run()'s decode and asserts the replaced text is
returned; the asyncssh regression test asserts errors="replace" is
passed through to asyncssh, which performs the decode in its channel
layer.

Note: this is one of five single-concern fixes in this batch touching repose/connection.py (several also repose/aiossh.py), on top of the four already-open connection PRs #182–#185. They conflict pairwise; any merge order works and later ones get a trivial rebase, which I'll provide.

Verification

Full gate green (ruff format/check, ty, pytest: 549 passed, coverage 83.49%). The paramiko regression test feeds non-UTF-8 bytes through the real decode; the asyncssh test pins the errors="replace" pass-through (the decode happens inside asyncssh's channel layer). Both verified to fail on the pre-fix code. Reviewed by a fan-out adversarial code review (two finder lenses per branch, two verifier lenses per finding); all confirmed findings were addressed before opening.

AI-assisted.