fix(sdk): harden agent-loop error recovery, retry, and pause semantics

[matej21]

Fixes the three high-severity findings from the agent-SDK architecture review (CORR-1..3), plus the two related mediums they subsume (CORR-6 crash-window message loss, CORR-10 lost manual pause).

Context

Git history shows this is one bug family patched case-by-case: 1cb6a25 introduced preserve-for-retry token semantics, then b2cfd17, c2220d6 and 245b2b1 each fixed one instance of the resulting resume_from_error ↔ infer loop (empty-response, aborted, non-retryable). This PR closes the family with three invariants instead of a fifth spot patch.

Invariant 1 — error-resume cycles back off (CORR-1)

A retryable LLM error (rate limit / server error / network / timeout) that outlives withLLMRetry's 5 attempts keeps the dequeue token unconsumed, so decide() re-entered infer immediately: an unbounded, backoff-free spin that pinned the loop and sent the parent one mailbox error message per cycle during a provider outage.

• AgentState now tracks consecutiveInferenceFailures / lastInferenceFailureAt (reducers: inference_failed increments, inference_completed resets, session_restarted resets the episode).
• continue()'s resume_from_error branch waits out an exponential backoff (5s base, 60s cap, per-agent errorResumeBackoff config) and yields instead of spinning; a timer re-schedules processing when the backoff elapses.
• onError hooks (parent notification) fire once per error episode, not once per cycle.
• Capped rather than ceilinged: the preserved message keeps retrying at the max delay until the provider recovers — dropping it would lose user work.

Invariant 2 — tokens are consumed only when the turn commits (CORR-2, CORR-6)

markConsumed ran before executeAfterInference, so a plugin returning {action:'retry'} recursed into runInference with the mailbox message already consumed: a pure-message turn stranded the agent in inferring (→ idle, unrecoverable), a tool-result turn double-appended pendingMessages and produced duplicate tool_result blocks that Anthropic rejects.

• Consumption moved to the commit points, after the retry decision is final.
• New inference_retried event rolls back the staged turn (clears pendingMessages, like the inference_failed rollback from 1cb6a25) so the retry re-collects the same unconsumed messages — nothing lost, nothing duplicated.
• Ordering is commit-then-consume: a crash in the gap now re-delivers the message on restart (at-least-once) instead of silently dropping it (session_restarted wipes staged pendingMessages, so consume-first lost the message).

Invariant 3 — pause is never clobbered mid-turn (CORR-3, CORR-10)

The tool_exec loop ran all pending tools without re-checking status, and the tool_completed/tool_failed/inference_completed reducers recomputed status unconditionally — a pause from a tool hook (or a manual pauseAgent() landing mid-inference) was silently overwritten, the paused tool re-executed its side effects, and the history could end with an unpaired tool_use.

• Reducers preserve paused; the tool_exec loop stops at the first pause (remaining tools stay pending and run after resume).
• afterToolCall pause still emits the executed tool's result event — side effects already ran, so the committed tool_use gets its tool_result and nothing re-executes on resume. (beforeToolCall pause keeps gate semantics: tool never started, runs after resume.)
• agent_resumed routes back to tool_exec while tool calls are pending — resuming into pending would have run inference with unresolved tool calls.

Tests

New error-recovery.integration.test.ts (6 tests): outage → backoff timing between failure/resume events, single parent notification, message survives and is delivered exactly once; afterInference retry for pure-message and tool-result turns (no stranding, no duplicate tool results in history or LLM request); beforeToolCall/afterToolCall pause (nothing runs while paused, exactly-once execution across resume); manual pause during in-flight inference (pause survives the commit). Also adds TestSession.pauseAgent/resumeAgent.

bun run ts:build clean, full sdk suite 1642 pass / 0 fail.

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https://claude.ai/code/session_01X2Dk23As1wEZ14mie22hGA