ssh.c: don't fail a channel read whose window credit is deferred - #1192

Problem

src/ssh.c has three channel read paths. _ChannelReadExt() already separates
"bytes were delivered" from "the window-adjust packet made it onto the wire".
The other two conflate them.

_ChannelRead() — behind wolfSSH_ChannelIdRead() and wolfSSH_ChannelRead() —
consumes the bytes, then returns the send result:

inputBuffer->idx += bufSz;                    /* bytes consumed */
updateResult = _UpdateChannelWindow(channel);
if (updateResult == WS_SUCCESS)
    updateResult = bufSz;
return updateResult;                          /* WS_WANT_WRITE -> "failed" */

_UpdateChannelWindow() returns WS_WANT_WRITE whenever the adjust would block —
routine on a non-blocking socket, not an error. The caller is told the read failed
after the data has already left the input buffer, so those bytes are gone, and a
caller using the usual if (cnt_r <= 0) break; shape tears the connection down
(src/wolfscp.c:1762 is one). wolfSSH_stream_read() has the same shape without the
loss — it skips inputBuffer->idx += n, so the data is re-delivered — but still
reports a failure for a read that copied n bytes.

SendPacket() sets ssh->error only for WS_WANT_WRITE, so a hard transport failure
during the adjust was invisible on both paths.

Fix (src/ssh.c)

Both paths now report the bytes copied and keep the send result out of band, matching
_ChannelReadExt():

• _ChannelRead() returns bufSz unconditionally; wolfSSH_stream_read()
  advances inputBuffer->idx before the adjust, then returns n.
• Both record a non-success adjust result in ssh->error and WLOG at
  WS_LOG_ERROR for anything other than WS_WANT_WRITE.
• _ChannelRead() also clears a stale WS_WANT_WRITE once the adjust does go out
  with the output buffer drained — its entry points, unlike wolfSSH_stream_read(),
  do not reset ssh->error.

A deferred credit still goes out: wolfSSH_worker() and the next SendChannelData()
flush pending output before doing anything else.

API note: wolfSSH_stream_read(), wolfSSH_ChannelRead() and
wolfSSH_ChannelIdRead() now return the byte count where a deferred or failed window
adjust previously produced a negative return. Callers that relied on that negative
return must check wolfSSH_get_error() after a successful read. Documented in
wolfssh/ssh.h and the wolfSSH_stream_read() block comment.

Tests (tests/unit.c)

One harness per path, each putting a full window of channel data and reading it back:

Verification

• unit.test, api.test, testsuite.test and scripts/{sftp,scp,fwd,get-put}.test
  all pass; unit.test also clean under ASan + UBSan.
• Clean under -Werror with gcc-13 across 6 configurations (enable-all, Zephyr
  defines, sftp-only, scp-only, default, small-stack).
• Negative controls: reverting the ssh->error recording fails both hard-failure
  assertions; reverting the stale-WS_WANT_WRITE clearing fails the third phase.

The echo server's worker loop has its own partial-write problems in this area. Those
are a separate rework PR that builds on this one — deliberately not in scope here.