FEATURE: SSE Resumption in execd

[Pangjiping]

Summary

Problem

• POST /command stream output over SSE; if the client disconnects, they lose the tail of the stream and cannot catch up or reconnect to the same execution cleanly.
• Need a stable, monotonic event id (eid) per stdout/stderr chunk so clients can request "everything after after_eid" without ambiguity.

What we changed (how it’s solved)

• pkg/sbuf: bounded ring-buffer store per stream id, keyed by eid, with strict monotonic checks - storage for replay after disconnect.
• command_stream + SSE path: for RunCommand / RunCode with resumeEnabled, the primary SSE registers a hub, and writeSSE both writes to the live connection and appends stdout/stderr frames (eid > 0) to the buffer; on request end, cleanup removes hub + buffer entry.
• GET /command/:id/resume: EventsAfter replays buffered SSE payloads, then if the command is still running and no primary SSE holds the live slot, tryAttachResume attaches the new response writer as the sole live consumer; 409 if the primary stream is still active.
• specs/execd-api.yaml: documents GET /command/{id}/resume, after_eid, responses (200 SSE, 409 via shared Conflict), aligned with the implementation.

Testing

• Not run (explain why)
• Unit tests
• Integration tests
• e2e / manual verification

Breaking Changes

• None
• Yes (describe impact and migration path)

Checklist

• Linked Issue or clearly described motivation. execd for SSE Resumption Between execd and SDKs #507
• Added/updated docs (if needed)
• Added/updated tests (if needed)
• Security impact considered
• Backward compatibility considered

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[ninan-nn]

LGTM : > I think this should be described as best-effort resume rather than lossless resumption. There are still at least two event-loss windows: concurrent stdout/stderr append can drop older eids due to the buffer’s strict monotonic check, and the replay-then-attach flow can miss events generated between the buffer snapshot and live handoff. So reconnect improves recoverability, but it does not yet guarantee gap-free continuation.