fix: a request the upstream refuses no longer fails over or trips breakers

[fylorn]

Summary

check_status wrapped every upstream non-2xx except 429/401/403 as ProviderError, and is_retryable treated ProviderError as retryable. A client's bad request (400) walked every route of the model, and every hop recorded a breaker failure, so one caller could open the breakers of all routes of a model. ProviderHttpError and ProviderTimeout were never constructed in production.

• check_status returns ProviderHttpError { status, .. }; reqwest timeouts become ProviderTimeout (send and whole-body read).
• routing::is_upstream_failure decides failover and breaker accounting:
  • fail over and count against the route: 5xx, 408, 429, timeouts, network errors, unreadable bodies, and 401/403 (UpstreamAuthError: the gateway's own credential for that route was refused, which another route may not share);
  • go straight back to the caller and count as the upstream working: any other 4xx. This matches the desktop gateway.
• Streams use the same rule in record_outcome (from the recorded error tag and status). A tool-inspection cut (PolicyBlocked) no longer counts against the route either, matching the buffered path.
• protocol_relearn no longer scrapes the status out of the message text. The probe's is_inconclusive does the same, and treats 5xx/408 as inconclusive.
• Doc comments in transport.rs and routing.rs rewritten to match.

A 5xx now reaches the caller with the upstream's status rather than a blanket 502.

Test plan

• Integration, new in gateway_failover.rs:
  • a_refused_request_goes_back_without_trying_another_route: two routes that both answer 400; the caller gets the 400 and its reason; only one upstream is hit;
  • refused_requests_do_not_open_the_breaker: four 400s (buffered and streamed) with min_samples = 2; the fifth request still reaches the upstream;
  • server_errors_open_the_breaker: two 500s open it, and the third request does not reach the upstream.
• Full integration suite locally (--ignored --test-threads=1)
• fmt, clippy --all-targets -D warnings and --lib, workspace unit tests

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