websocket: fix message send corruption

- Fix a bug in EAGAIN handling when sending frames that led to a
  corrupted last byte of the frame sent.

- Restore sanity to curl_ws_send() behaviour:

  - Partial writes are reported as OK with the actual number of
    payload bytes sent.

  - CURLE_AGAIN is only returned when none of the payload bytes
    (or for 0-length frames, not all of the frame header bytes)
    could be sent.

  - curl_ws_send() now behaves like a common send() call.

- Change 'ws-data' test client to allow concurrent send/recv
  operations and vary frame sizes and repeat count.

- Add DEBUG env var CURL_WS_CHUNK_EAGAIN to simulate blocking
  after a chunk of an encoded websocket frame has been sent.

- Add tests.


Prior to this change data corruption may occur when sending websocket
messages due to two bugs:

1) 3e64569a (precedes 8.10.0) caused a data corruption bug in the last
   byte of frame of large messages.

2) curl_ws_send had non-traditional send behavior and could return
   CURLE_AGAIN with bytes sent and expect the caller to adjust buffer
   and buflen in a subsequent call. That behavior was not documented.


Reported-by: na-trium-144@users.noreply.github.com

Fixes https://github.com/curl/curl/issues/15865
Fixes https://github.com/curl/curl/issues/15865#issuecomment-2569870144
Closes https://github.com/curl/curl/pull/15901

docs/libcurl/libcurl-env-dbg.md

@@ -130,6 +130,11 @@ greater. There is a number of debug levels, refer to *openldap.c* comments.
 Used to influence the buffer chunk size used for WebSocket encoding and
 decoding.
 
+## CURL_WS_CHUNK_EAGAIN
+
+Used to simulate blocking sends after this chunk size for WebSocket
+connections.
+
 ## CURL_FORBID_REUSE
 
 Used to set the CURLOPT_FORBID_REUSE flag on each transfer initiated