fix(llm): retry deepseek rounds on zero-progress transport failures

A single undici "TypeError: terminated" (mid-stream TLS reset) killed a
40-minute flash lab run at top level - the deepseek adapter had no retry
at all, unlike the openai/claude-p lanes. Wrap each round's request +
SSE consumption in a classified retry (3 attempts, 2s/4s backoff):
transport-class errors only (socket/DNS errnos, undici terminated/fetch
failed, HTTP 429/5xx), and only while zero deltas have been forwarded -
consumers accumulate deltas into the persisted turn, so post-emission
retries would duplicate text and resampled output cannot be stitched.
The exposed window is dominated by silent prefill on large contexts,
which is exactly the zero-progress case. Retries are trace-recorded as
"transport_retry". Semantic errors (4xx, provider chunk.error) stay
fatal; classifier probed against the captured error shapes (10/10).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pfUVhNFTRvhYGXBwoKNj6