test(ecstore): fix rename_all WARN flake from callsite-interest poisoning

rename_all_missing_source_still_warns and rename_all_real_failure_still_warns
assert that `warn_reliable_rename_failure` emitted its WARN, but that is a
single production callsite shared with tests that call rename_all *without*
installing a subscriber — rename_all_missing_source_returns_file_not_found,
two tests above, is one of them.

tracing caches each callsite's Interest process-globally and the first thread
to reach a callsite fixes that value; while at most one dispatcher is
registered, tracing-core derives it from the registering thread's own
subscriber, and registration is once-only. When the subscriber-less sibling
wins, the callsite is cached as Interest::never() and the WARN never fires,
so the assertion sees empty output:

    ordinary missing-source failures must keep the WARN, got:

Reproduced at 3/25 with `disk::os::tests::rename_all_missing_source` (both
tests), against 0/20 for the victim alone. Fixed by pinning callsite interest
inside warn_capture(), so every current and future user of that helper is
covered rather than just the two tests that happen to fail today.

pin_callsite_interest_for_test() moves from cluster::rpc::background_monitor
to a new crate-level test_tracing module: it is domain-neutral and now has
consumers in two unrelated subsystems, and disk::os should not have to reach
into a cluster::rpc test helper.

Verified: repro filter 0/30 (was 3/25); disk::os:: 0/12; cluster::rpc:: 0/12
and its poisoner pair 0/15, confirming the moved helper still holds.

Follow-up to #5438. Closes the last item in #5439.

crates/ecstore/src/disk/os.rs

@@ -771,9 +771,26 @@ mod tests {
         }
     }
 
+    /// Holds a `warn_capture()` capture alive: the thread-local subscriber, plus
+    /// the pin that keeps tracing's process-global callsite-interest cache from
+    /// being decided by some other test's thread.
+    struct WarnCaptureGuard {
+        _subscriber: tracing::subscriber::DefaultGuard,
+        _callsite_pin: tracing::Dispatch,
+    }
+
     /// Capture WARN-level output on the current thread; tokio tests here run on
     /// the current-thread runtime, so the guard covers the whole test body.
-    fn warn_capture() -> (CapturedLogs, tracing::subscriber::DefaultGuard) {
+    ///
+    /// The callsite pin matters because `warn_reliable_rename_failure` is a
+    /// single production callsite shared with tests that call `rename_all`
+    /// *without* installing a subscriber — `rename_all_missing_source_returns_file_not_found`
+    /// is one. Whichever thread reaches it first fixes its `Interest`
+    /// process-wide, so without the pin that sibling can cache
+    /// `Interest::never()` and the WARN never fires here at all, leaving the
+    /// "must keep the WARN" assertions staring at empty output. See
+    /// [`crate::test_tracing::pin_callsite_interest_for_test`].
+    fn warn_capture() -> (CapturedLogs, WarnCaptureGuard) {
         let logs = CapturedLogs::default();
         let subscriber = tracing_subscriber::fmt()
             .with_max_level(tracing::Level::WARN)
@@ -781,7 +798,10 @@ mod tests {
             .with_ansi(false)
             .without_time()
             .finish();
-        let guard = tracing::subscriber::set_default(subscriber);
+        let guard = WarnCaptureGuard {
+            _subscriber: tracing::subscriber::set_default(subscriber),
+            _callsite_pin: crate::test_tracing::pin_callsite_interest_for_test(),
+        };
         (logs, guard)
     }