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Anthropic streaming path (gateway_service.go) returned a plain error on
upstream SSE read failure, so the handler-level UpstreamFailoverError check
never fired and the client received a bare `stream_read_error` event,
breaking long-running tasks even when no bytes had been written yet.

The most common trigger is HTTP/2 GOAWAY from api.anthropic.com edge
backends doing graceful rotation: Go's http.Transport surfaces this as
`unexpected EOF` and never auto-retries.

Mirror what the OpenAI and antigravity gateways already do: when the read
error happens before any byte has reached the client (`!c.Writer.Written()`),
return `*UpstreamFailoverError{StatusCode: 502, RetryableOnSameAccount: true}`
so the handler can retry on the same or another account. After client
output has begun, SSE has no resume protocol — keep the existing passthrough
behavior.

Tests cover both branches via streamReadCloser-based fixtures.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>