fix(agents): surface fallback model failures with actionable details

[w7-mgfcode]

Sub-issue of #380 (umbrella: fix(repo): platform reliability hardening — agents, config, ui, forecast). Parallel after Foundation (E1 #334).

Purpose

Surface agent fallback-model failures with actionable, classified details (model-not-found / quota / auth) over REST chat and the /agents/stream WebSocket error events, instead of a generic failure.

Sub-tasks

To be decomposed via issue-to-subtasks when this epic is picked up.

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Summary

When every model in the FallbackModel chain fails, the frontend shows only a generic Stream error: All models from FallbackModel failed (2 sub-exceptions). The backend logs contain the actionable per-model causes — these should be surfaced (safely) to the API/WebSocket client so issues are diagnosable from the UI.

Observed

Frontend (experiment agent, streaming chat):

Error: Stream error: All models from FallbackModel failed (2 sub-exceptions)

Backend agents.websocket_stream_error log held the real causes:

• Primary — ModelHTTPError 404: invalid model name (models/google-gla:gemini-3-flash-preview is not found).
• Fallback — ModelHTTPError 429: quota exhausted (RESOURCE_EXHAUSTED, free-tier generate_content_free_tier_requests limit 20/day for gemini-2.5-flash).

Expected

The WebSocket error event (and the equivalent REST chat error) should preserve a safe summary of each fallback sub-failure, e.g. per-model { model_name, status_code, reason }:

• 404 → "model not found / invalid model name"
• 429 → "quota/rate limit exhausted"
• 401/403 → "authentication/permission error"

So the UI can distinguish a model-name problem from a quota problem without a maintainer reading container logs.

Constraints

• Never expose secrets — no API keys, bearer tokens, Authorization headers, or AIza… values in the surfaced summary or logs. Include only status code + provider message text (which is secret-free) or a mapped reason string.
• Keep the RFC 7807 / ErrorEvent shape; classification should be additive.

Tests

• A FallbackExceptionGroup with mixed sub-errors (404 + 429) produces an error payload listing each model's safe reason.
• Assert no secret-like material leaks into the surfaced error.

Context

Found while investigating the experiment-agent stream failure on 2026-06-01. Companion issue covers rejecting the doubled provider prefix that caused the 404 leg.

[w7-mgfcode]

Shipped in PR #383 (merged to dev). All configured-model provider failures now surface as classified, secret-safe details: WS error event with error_type=fallback_exhausted + failures[], REST chat 502 AGENT_FALLBACK_EXHAUSTED problem+json with the failures extension member.