Add process diagnostics snapshots (#37434)

## What changed

- Add a `codex-diagnostics` crate that snapshots the process ID, available
  resident-memory measurements, and registered process-wide gauges.
- Provide guards that update gauges for the lifetime of measured objects.
- Track live `CodexThread` instances with the `core.threads.live` gauge.

## Testing

- Add unit coverage for gauge registration, guard lifetimes, process memory
  snapshots, and live-thread reporting.

GitOrigin-RevId: 3236b086bd4ebe31ed4768ab87a5fa288b0891b0

codex-rs/diagnostics/src/tests.rs

@@ -0,0 +1,58 @@
+use super::Gauge;
+use super::snapshot;
+use pretty_assertions::assert_eq;
+
+static GUARD_GAUGE: Gauge = Gauge::new("diagnostics.tests.guard");
+static REGISTERED_GAUGE: Gauge = Gauge::new("diagnostics.tests.registered");
+
+#[test]
+fn gauge_guards_follow_the_measured_lifetime() {
+    let first = GUARD_GAUGE.track();
+    let second = GUARD_GAUGE.track();
+    let value = || {
+        snapshot()
+            .gauges
+            .into_iter()
+            .find(|gauge| gauge.name == "diagnostics.tests.guard")
+            .expect("used gauge should be registered")
+            .value
+    };
+
+    assert_eq!(value(), 2);
+    drop(first);
+    assert_eq!(value(), 1);
+    drop(second);
+    assert_eq!(value(), 0);
+}
+
+#[test]
+fn snapshot_includes_process_memory_and_registers_gauges_once() {
+    REGISTERED_GAUGE.increment();
+    REGISTERED_GAUGE.increment();
+    let diagnostics = snapshot();
+    let registered = diagnostics
+        .gauges
+        .iter()
+        .filter(|gauge| gauge.name == "diagnostics.tests.registered")
+        .collect::<Vec<_>>();
+
+    assert_eq!(diagnostics.process.id, std::process::id());
+    assert_eq!(registered.len(), 1);
+    assert_eq!(registered[0].value, 2);
+    #[cfg(any(target_os = "macos", target_os = "linux", target_os = "windows"))]
+    assert!(
+        diagnostics
+            .process
+            .resident_memory_bytes
+            .is_some_and(|bytes| bytes > 0)
+    );
+    #[cfg(target_os = "macos")]
+    assert!(
+        diagnostics
+            .process
+            .physical_footprint_bytes
+            .is_some_and(|bytes| bytes > 0)
+    );
+    #[cfg(not(target_os = "macos"))]
+    assert_eq!(diagnostics.process.physical_footprint_bytes, None);
+}