When a multi-thread source ignores ranged reads, abort the partial
destination and retry the copy as a single stream. Reset accounting
before the fallback so transfer progress remains accurate.
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.