test(#2862): cover both fd/op mismatch branches in hive-priv

check_fd_agreement is the guard that keeps a descriptor and the request
it arrived with in agreement, and both of its rejections were untested.

An fd-taking op with no descriptor must not fall back to anything: a
temp file or the response socket would send an agent's state somewhere
the caller never asked for. The mirror case matters for a different
reason -- returning the error is what drops the OwnedFd and closes it,
so ignoring a stray descriptor instead would leak one per bad request
in a long-lived root process.

The third test pins both agreeing combinations, so the check is
rejecting mismatches rather than descriptors in general. Descriptors
are real /dev/null handles so the closing drop is genuinely exercised.

hive-priv/src/main.rs

@@ -2486,12 +2486,68 @@ async fn sync_agent_tmpfiles(agents: &[String]) -> Result<(String, String)> {
 #[cfg(test)]
 mod tests {
     use super::{
-        PAUSED_MARKER_FILE, limits_dropin_body, redact_password_line, remove_marker_in,
-        write_state_file_nofollow,
+        OwnedFd, PAUSED_MARKER_FILE, PrivRequest, check_fd_agreement, limits_dropin_body,
+        redact_password_line, remove_marker_in, write_state_file_nofollow,
     };
     use std::path::PathBuf;
     use std::sync::atomic::{AtomicU32, Ordering};
 
+    /// A request that streams into a caller-supplied descriptor.
+    fn fd_taking_request() -> PrivRequest {
+        PrivRequest::SendAgentSnapshotToFd {
+            agent_name: "atlas".to_owned(),
+            snapshot_name: "hive-migrate".to_owned(),
+            parent_snapshot_name: None,
+        }
+    }
+
+    /// A real descriptor — `/dev/null` rather than a fake, so the drop
+    /// that closes it on a rejection path is genuinely exercised.
+    fn some_fd() -> OwnedFd {
+        std::fs::File::open("/dev/null")
+            .expect("open /dev/null")
+            .into()
+    }
+
+    /// An op that streams into a passed descriptor cannot invent one:
+    /// falling back to anything (a temp file, the response socket) would
+    /// send an agent's state somewhere the caller never asked for.
+    #[test]
+    fn an_fd_taking_op_without_a_descriptor_is_rejected() {
+        let err = check_fd_agreement(&fd_taking_request(), None)
+            .expect_err("no descriptor arrived, so this must not proceed");
+        let msg = format!("{err:#}");
+        assert!(msg.contains("requires a passed file descriptor"), "{msg}");
+    }
+
+    /// The mirror case: a descriptor sent alongside an op that takes
+    /// none is a protocol error, not something to ignore. Returning the
+    /// error drops the `OwnedFd`, which closes it — the alternative
+    /// leaks one descriptor per stray request in a long-lived root
+    /// process.
+    #[test]
+    fn a_descriptor_sent_to_an_op_that_takes_none_is_rejected() {
+        let fd = some_fd();
+        let err = check_fd_agreement(&PrivRequest::DaemonReload, Some(&fd))
+            .expect_err("an unexpected descriptor must not be silently ignored");
+        let msg = format!("{err:#}");
+        assert!(
+            msg.contains("does not take a passed file descriptor"),
+            "{msg}"
+        );
+    }
+
+    /// Both agreeing combinations pass, so the check rejects mismatches
+    /// rather than descriptors in general.
+    #[test]
+    fn agreeing_combinations_are_accepted() {
+        let fd = some_fd();
+        check_fd_agreement(&fd_taking_request(), Some(&fd))
+            .expect("an fd-taking op with its descriptor is the normal case");
+        check_fd_agreement(&PrivRequest::DaemonReload, None)
+            .expect("every ordinary request arrives without a descriptor");
+    }
+
     /// An unset weight is "not configured": the drop-in must come out
     /// byte-identical to the pre-weights two-setting body, so a hive-c0re
     /// older than this field can't change what lands on disk.