Keep pooled SFTP connection on application-level SFTP errors

[stegololz]

SFTPClientPool.get_sftp_client yields the pooled client inside a bare
except Exception and marks the connection faulty for any exception the caller's
async with body raises, including SFTP status replies such as SFTPNoSuchFile
or SFTPPermissionDenied.

A status reply means the server answered over a live channel, so the connection is
still healthy. Dropping it costs a reconnect and re-auth each time, and since the
caller usually keeps iterating, one transient miss becomes self-sustaining.

Observed on a production task iterating ~36,000 files over one pool:

phase	files	SSH connections
before first miss	28,380	23
after first miss	7,598	9,244, with 7,598 re-auths

The files were not missing; they read back fine afterwards. Each failure also
logged a full traceback, producing 135-190 MB of task logs per attempt.

Only SFTPNoConnection, SFTPConnectionLost and SFTPBadMessage now count as
transport faults. Every other SFTPError returns the connection to the pool
intact. Non-SFTPError exceptions and CancelledError keep the previous
behaviour and still drop it. The exception always propagates unchanged.

The rule sits in a module-level _is_connection_faulty rather than inline in the
except block because it is a pure function of the exception type: one
parametrised test covers the whole SFTPError taxonomy with no pool, event loop
or mocked connection to stand up. It also keeps get_sftp_client reading as
acquire, classify, log, release, and puts the rule beside the
_TRANSPORT_SFTP_ERRORS tuple it consults.

The pool was introduced in #64465.

Tested with pytest providers/sftp/tests/unit/sftp/pools/test_sftp.py: 32 passed,
covering the classifier table, a miss returning the connection to the pool, five
consecutive misses reusing the same client without reconnecting, and the transport
and non-SFTPError cases still dropping it.

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