fix(models): status-less stream faults classify for every provider family

[rezaho]

What

A mid-stream SSE fault arrives under HTTP 200 — no status code — and the classification tree gated everything on elif status_code:, with the only status-less arm restricted to Anthropic. An OpenAI/Azure in-stream fault therefore kept the defaults: UNKNOWN, is_retryable=False, and the literal message "API Error". A caller's retry ladder sees a terminal verdict for the most retryable faults a provider emits (an in-stream rate limit included).

Two halves, either alone leaving the bug alive (proven by driving each half separately):

1. adapters/streaming.py — the Responses error event is flat (code/message on the event itself, per the generated ResponseErrorEvent type); it was read with Anthropic's nested shape, which returned {} and replaced the provider's verdict with a synthetic {"type": "unknown"}. It now normalizes the flat grammar into the same {code, message} shape the response.failed arm already yields.
2. agents/exceptions.py — the status-less arm now hoists message/type/code extraction above a provider dispatch (the provider's real words survive for every provider, mapped or not) and classifies the openai/azure family by its code vocabulary (server_error/bare response.failed → SERVICE_UNAVAILABLE retryable; rate_limit_exceeded/rate_limit_error → RATE_LIMIT retryable; insufficient_quota → INSUFFICIENT_CREDITS non-retryable). bedrock joins the anthropic branch — the same share the status-code arm already makes; without it every Bedrock in-stream fault is unclassified-terminal.

Deliberately unchanged, pinned by tests

The synthetic max_retries and incomplete_stream markers stay UNKNOWN/non-retryable, and unmapped codes default to non-retryable. Flipping any of these is a policy decision, tracked separately.

Notes for review

• The widened arm sits before the transport-classification arm; a caller passing both exception= and a status-less {"error": ...} dict would take this arm. No call site does both today (audited: anthropic.py, anthropic_oauth.py, openai.py are the only status-less-dict callers; google/openrouter/local pass response objects; openai_oauth stamps a synthetic 500).
• Tests cover three layers: the accumulator grammar, arun_streaming end-to-end over the fake session (classification + retryable + the provider's words), and the harmonize classification table parameterized across provider families.
• This upholds the contract already stated at tests/models/test_adapter_harmonize.py: every stream outcome maps to either a valid HarmonizedResponse or a typed, classified ModelAPIError.