fix(provider): retry or fall back after schema-invalid 2xx responses with no choices

[Audacity88]

Summary

A schema-invalid 2xx chat-completion response with no choices fails immediately as a non-retryable decode error. The configured retry count and fallback chain are not used, even though no output was emitted and the malformed response may be a transient proxy/model failure.

Reproduction

During a non-streaming free-model review run:

zoder exec --oneshot --approve none --no-stream \
  --max-tokens 4000 \
  --model nvidia/nemotron-3-ultra-550b-a55b:free \
  --require-free \
  < prompt.md

Zoder routed successfully through an OpenAI-compatible OpenRouter provider, attempted the real request, and returned:

zoder: error: malformed chat-completion response: no completion choices

The same route had completed successfully in an earlier run. No model output was emitted before the failure.

Current behavior

consume_full correctly rejects an empty choices array as ErrKind::Decode. However, ProviderError::retryable only treats timeout, rate-limit, server, and network errors as retryable. The one-shot dispatch loop therefore does not use its configured retries for this response shape.

This is correct fail-closed behavior for the individual response, but it turns a potentially transient empty proxy response into an immediate terminal transport failure.

Expected behavior

When a 2xx response has no meaningful completion choice and no output has been emitted, Zoder should make a bounded recovery attempt before failing the run. Possible behavior:

1. Retry the same model/provider up to the configured retry count.
2. If it remains schema-invalid, continue to the configured fallback chain when policy permits.
3. Preserve the current terminal decode error when retries/fallbacks are exhausted.

The diagnostic should retain the selected provider/model and enough safe response metadata to distinguish a transient empty response from a permanently incompatible wire format without exposing credentials or arbitrary response content.

Environment

• zoder 0.2.1
• OpenAI-compatible OpenRouter provider
• Model: nvidia/nemotron-3-ultra-550b-a55b:free
• Non-streaming one-shot execution