Drop dead read_error_body_or_default / LAYER_RUNTIME; inline single-use
_build_default_credential/_safe_close; compact incident narratives to their
invariants in the guards. Byte-cap and deadline semantics of
read_streaming_error_body verified identical.
Addresses @helix4u's review on #91493:
- conversation_loop now stamps failure_retryable (the real ClassifiedError
verdict) next to failure_reason; error_surface prefers it and only falls
back to the reason set for older results. Fallback set corrected to match
classify_api_error (auth, format_error, billing_unverified now
non-retryable).
- The descriptor carries the failing session's provider/model captured at
classification time; Copy error details prefers them over the foreground
composer atoms.
- Open logs is labeled 'Open Desktop logs' on remote/cloud connections —
the local folder holds transport logs, not the remote runtime's.
- API-exception module allowlist widened to botocore/boto3/google/grpc/
requests/aiohttp so other adapter SDKs don't misclassify as gateway.
Turn errors now carry a structured {layer, code, retryable} descriptor
(agent/error_surface.py) built from the same classifier the retry loop
uses. The tui_gateway stamps it on terminal error frames, retained
failed-turn snapshots, and resume replay; the Desktop error card renders
the layer title (provider / endpoint / streaming / auth / billing /
gateway / runtime / disk) plus matched actions: Retry, Switch provider,
Open logs, Copy diagnostics.
Older backends that omit the descriptor keep today's behavior (generic
title, string-sniff fallbacks) — the field is advisory on both sides.
