Validate ranged GET responses before returning their bodies. Close invalid
responses, retry malformed partial responses through the pacer, and return
without retrying when a server deterministically ignores Range.
Fixes#6980
Add response validation for calls made with Range open options. Verify
Content-Range, Content-Length, response status, and the complete
representation size before a backend accepts the response body.
Return a shared sentinel when a server ignores a partial range so callers
can avoid retrying the same unsupported request.