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fix: unify bb_data_http's SSE send-failure contract as fatal-abort, WS as retriable (B1-1424, B1-1429) (#1230)

The SSE connect-time preamble write and the flush loop's send-failure
handling were fixed separately and landed on incompatible contracts: the
preamble fix made a failed write non-fatal (log + fall through), while
the flush loop retried EVERY send_fn failure identically regardless of
transport. Both are unsafe for SSE: bb_http_resp_send_chunk() wraps
ESP-IDF's httpd_resp_send_chunk(), whose first call on a response issues
several separate httpd_send_all() writes (status line, headers,
end-of-headers CRLF, chunk-size line, body, trailing CRLF) and only
marks the response's internal first_chunk_sent AFTER all of them
succeed; httpd_send_all() discards the partial byte count on a failed
write, so a cold-socket EAGAIN mid-header-block can leave some bytes
already on the wire with first_chunk_sent still false. Retrying (or
"logging and continuing") after that failure lets the next chunk write
re-send the entire status line + header block on top of what is already
there -- a connection that looks healthy but delivers duplicated/garbled
headers interleaved with body bytes forever, with no resync token in
HTTP chunked encoding to recover. Retrying an SSE send failure is unsafe
at every position, not just the connect-time preamble's.

This collapses both fixes into one coherent contract instead:

- The connect-time SSE preamble write is REMOVED outright (not made
  non-fatal): it is purely cosmetic, nothing downstream depends on
  receiving it, and ESP-IDF httpd commits response headers on whichever
  chunk write happens first -- preamble or the broadcaster's own first
  real frame. The three pre-existing connect-time setsockopt() calls now
  check their return values and log a warning on failure (previously
  unchecked).
- bb_data_http_send_fn's return contract (bb_data_http.h) now
  distinguishes RETRIABLE (BB_ERR_TIMEOUT -- send_fn certifies nothi... (continued)

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