fix(anthropic): ping-aware stream watchdog — eliminate spurious NonZeroAgentExitCodeError

[ericleepi314]

Summary

Fixes the dominant failure class in the terminal-bench 2.1 eval: 18/89 trials died with NonZeroAgentExitCodeError (clawcodex mean 0.58 vs claude-code 0.72, which had zero such crashes on the same model).

Root cause (verified at SDK/httpx source)

The Anthropic Python SDK drops ping keepalive events (_streaming.py:151 if sse.event == "ping": continue), so the idle watchdog — iterating the typed event stream — sees dead air whenever the model works >90s between typed events. In the agentic loop every turn re-sends the growing conversation, so every call pays prompt-processing time-to-first-event, scaling toward minutes near opus-4-8's 1M window (the #730 1M fix made this common). Crash duration_ms clustered at 94–100s = just past the flat 90s window. The watchdog's recovery re-issued non-streaming, which the SDK refuses for opus-class max_tokens ("Streaming is required…") → fatal exit 1.

Fix (TS-parity, three layers)

1. Ping-aware liveness (primary): the dropped pings are still bytes that httpx.Response.num_bytes_downloaded counts (_models.py:952, per raw chunk in iter_bytes; the SDK consumes response.iter_bytes()). Each deadline re-arms instead of firing when bytes advanced → fires only on true dead air. Zero changes to the accumulation path.
2. Retry the stream (stream_idle_max_attempts, default 3) → StreamIdleTimeout on exhaustion. Fatal _fallback_to_chat deleted.
3. Two-phase deadline (300s first-event grace / 90s inter-event) as the time-based fallback.
   Plus explicit chat() timeout (API_TIMEOUT_MS, default 600s) which disarms the SDK's >10-min guard for legit non-streaming callers.

Test plan

• Live re-smoke, 6 tasks that all died with NonZeroAgentExitCodeError in the 0.58 run (opus-4-8 + subscription + effort high): regex-log, dna-assembly, feal-linear-cryptanalysis → 1.0 PASS; filter-js-from-html, adaptive-rejection-sampler → complete, reward 0.0, no exception (genuine misses); write-compressor → AgentTimeoutError (legit slow, same mode claude-code hits). Zero spurious watchdog crashes remain.
• 119 passing across watchdog/provider/ch04/thinking/query/context suites (byte-progress re-arm + dead-air fire, retry-then-succeed, exhaustion-raises, chat timeout, two-phase transition)
• Critic review (4 rounds) → "Ship the branch"

(Clean re-base of #733, which had a stale merge-base after main history was rewritten.)

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