docs: note Go SDK retry cancellation preserves HTTP error

[inference-gateway-maintainer]

Summary

The Go SDK retry layer changed how it reports errors when a request's context is cancelled while it is sleeping between attempts (inference-gateway/sdk PR 119). The returned error now wraps both the cancellation and the underlying transport/HTTP failure instead of collapsing to a bare context.Canceled / context.DeadlineExceeded.

Changes to sdks.md (Go SDK -> Retry and backoff):

• Added a "Cancellation preserves the underlying error" note explaining that on cancellation mid-retry the error keeps the failure that triggered the backoff.
• Documented that errors.Is(err, context.Canceled) / context.DeadlineExceeded still report the cancellation, and the underlying HTTP status stays visible in the message and via errors.Is / errors.As, including the "(cancelled while retrying: ...)" wrap format and a short code example.

Verified with lint:md, format:check, and build; the note is server-rendered into the built HTML.

Closes #381