fix: retry transient provider overloads in stream

[lawyer112]

What changed

• Preserve provider HTTP status codes in typed errors.
• Retry 429, 502, 503, and 504 responses inside the same RunSSE stream.
• Retry only when the failed provider pass produced no text, reasoning, finish reason, or tool invocation.
• Use bounded backoff delays of 2s, 5s, and 10s, with a maximum of three retries.

Why

Transient provider overloads currently terminate the local Cursor stream immediately. The client then sees an avoidable failed turn even though retrying before any output has been emitted is safe.

Impact

Short provider overloads can recover transparently without duplicating partial output or tool calls. Permanent errors and failures after output still fail normally.

Validation

• go test ./...
• Production candidate built and verified locally on the formal 18080/18090 runtime.