fix(llm): tolerate relay keepalive events in Responses stream folding

[3aKHP]

Problem

Second pelican test after #253 failed with Responses 流畸形：未知的语义事件 keepalive。 The relay emits keepalive SSE frames during long generations (builtin image generation in this case); the frame carries a global sequence_number (it occupies a slot in the upstream counter), but its type is unknown to the folder, so unknown-type fail-closed killed the reply.

Fix

Add keepalive to _RELAY_EVENTS — the existing home for relay-private structural events, gated behind the codex-http-relay tolerate_relay_events capability bit. Since the frame carries a proper sequence number, strict +1 validation continues seamlessly after the skip. The public profile still rejects it (official endpoints don't send it).

Tests

Two new: relay profile folds a stream with interleaved keepalives; public profile still fail-closes with 未知. Full suite 2443 passed, ruff clean.

[khpilot]

Bot Review — ✅ Approve

The PR adds keepalive to _RELAY_EVENTS so that the Responses stream folder tolerates relay-emitted keepalive SSE frames (which carry a proper sequence number) without killing the reply, while the public profile still fail-closes on unknown event types. Both reviewers completed their coverage with no blocking findings, and the fix is targeted and well-tested (relay profile passes keepalives, public profile still rejects them). Verdict: approve.

No findings reported.

Review provenance

• General reviewer: complete — 2/2 units completed
• Style reviewer: complete — 2/2 units completed
• Coverage: complete
• Reviewed head: 84ce793 (base 4bbf11f)

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