fix(kiro): preserve opaque reasoning activity through stream translation

[diegosouzapw]

Kiro can send native reasoningContentEvent frames containing an opaque signature before any text. The executor previously consumed those frames without forwarding a progress signal, so the downstream content-stall watchdog could abort an active reasoning stream.

Preserve that activity as an empty reasoning_content delta only for native reasoning events with a nonblank signature. The signature is never exposed or counted as generated content. Existing text extraction is moved to a small helper; metadata, empty/unknown signatures, timeout limits, cancellation, EOF policy and account health keep their existing behavior.

Validation:

• Two RED runs reproduce a content stall at the 100 ms budget while the text control passes. The unchanged initial regression then passes with signature activity spanning more than three budgets before the final answer.
• The expanded matrix passes 12/12, covering native and inline reasoning, fragmented frames, negative progress cases, stopped progress, active timeout and cancellation. The corrupt-frame case explicitly enables full CRC verification.
• Across 83 related files, 576/583 cases passed initially. The seven failures belonged to two local-HTTP files blocked by the first test harness; rerunning those files with explicit loopback allowance passed 14/14 (seven overlapping passes). This gives 583 unique passing cases across the recorded runs, not a single all-green broad run.
• Core/dashboard type checks, lint, both complexity gates, file-size, test discovery, formatting and docs checks pass. Independent code reviews found no blockers; normal commit hooks passed.
• The changelog gate fails only on the inherited CodeBuddy fragment, fixed on the newer base by the concurrently merged fix(changelog): turn the codebuddy-intl fragment into a bullet line #16118. i18n has the same pre-existing 23 source, 132 target and 132 stale findings as the pure base. The new paragraph is translated in all 66 mirrors; only their target hashes/timestamps are updated, preserving historical source hashes. No native-speaker review is claimed.

The reproduction uses synthetic AWS frames through the real executor/readiness/pipe path with mocked fetch and isolated storage. It does not certify a live Kiro account, HTTP/chatCore/combo/cooldown, or native terminal proof. A separate configurable content-stall timeout remains an optional issue follow-up outside this patch.

Refs #15260. Keep the issue open for merge and the remaining scope disposition.

[github-actions]

CI Coverage Report

• Coverage job: skipped
• PR test policy: success

Coverage artifact was not available for this run.