fix(test): make test_schedstat_fd_closed_on_thread_exit not flaky

[rcoh]

Summary

test_schedstat_fd_closed_on_thread_exit (added in 5e905e8) was flaky under the parallel test suite — failing roughly 3/10 runs of cargo test -p dial9-tokio-telemetry --lib, but passing every time when run in isolation.

Root cause. The test spawned a thread, called SchedStat::read_current() (which opens /proc/self/task/<tid>/schedstat and stores the OwnedFd in a thread-local), captured the raw fd number, joined, and then asserted fcntl(fd, F_GETFD) != -1. The kernel readily recycles fd numbers, and other tests in the same binary open many files concurrently — including their own per-thread schedstat fds opened from tokio worker poll callbacks (runtime_context.rs). If a sibling test opened any file in the small window after our spawned thread's OwnedFd was dropped, the fd number got reused and fcntl(F_GETFD) returned success on a totally unrelated fd, falsely reporting a leak.

Fix. Inside the spawned thread, also record what the fd points at via readlink("/proc/self/fd/<fd>"). After join, check whether the fd is either closed (readlink fails) or now points at a different file (slot was reused for an unrelated open — which itself proves our OwnedFd was dropped). The captured path /proc/self/task/<spawned_tid>/schedstat is unique to the spawned thread's open: each thread opens schedstat for its own tid, so no concurrent test can possibly hold an open fd for that exact path. If the fd is still open AND still points at it, that's a real leak — the assertion still fires.

The schedstat fd-cleanup invariant is unchanged; only the way the test observes it is hardened.

Test plan

• Reproduce the flake on main: 3/10 full-suite runs failed.
• Apply the fix and run the full suite 13 times in a row — all 228 tests pass every time.
• Verify the test still catches a real leak by temporarily replacing *cell.borrow_mut() = Some(owned) with std::mem::forget(owned) in the production path: test fails with schedstat fd 3 leaked after thread exit (still points at "/proc/.../task/.../schedstat"). Reverted before commit.
• cargo clippy -p dial9-tokio-telemetry --lib --tests clean.