* volume: validate replica upload targets in FetchAndWriteNeedle
The replica leg forwarded the fetched needle to a caller-supplied address
without checking it, so a malformed target could redirect the upload to an
unintended host or path. Require each replica target to be a bare host:port
whose host is not loopback / link-local / unspecified, reusing the address
deny-list; cluster peers legitimately sit on private networks, so RFC 1918 /
CGNAT stay allowed and -volume.allowUntrustedRemoteEndpoints still opts out.
Validate every target up front so a bad one fails the request before the local
write, and upload through a client that re-checks the resolved address at
connect time so a replica hostname cannot rebind to a blocked address after
validation. Mirrored in Rust (validation moved ahead of the local write; the
Rust S3 path's connect-time re-check is still a follow-up there).
* volume: only accept inline gcs credentials in FetchAndWriteNeedle
The gcs credentials value on this request could name a local filesystem path,
which the SDK reads from disk. Accept only inline JSON here; the server-side
GOOGLE_APPLICATION_CREDENTIALS env var still supplies a path. The Rust volume
server has no gcs backend, so there is nothing to mirror.