chore(otel): rename OtelManager to SessionTelemetry (#13808)

## Summary
This is a purely mechanical refactor of `OtelManager` ->
`SessionTelemetry` to better convey what the struct is doing. No
behavior change.

## Why

`OtelManager` ended up sounding much broader than what this type
actually does. It doesn't manage OTEL globally; it's the session-scoped
telemetry surface for emitting log/trace events and recording metrics
with consistent session metadata (`app_version`, `model`, `slug`,
`originator`, etc.).

`SessionTelemetry` is a more accurate name, and updating the call sites
makes that boundary a lot easier to follow.

## Validation

- `just fmt`
- `cargo test -p codex-otel`
- `cargo test -p codex-core`

codex-rs/core/src/tasks/compact.rs

@@ -30,14 +30,14 @@ impl SessionTask for CompactTask {
     ) -> Option<String> {
         let session = session.clone_session();
         let _ = if crate::compact::should_use_remote_compact_task(&ctx.provider) {
-            let _ = session.services.otel_manager.counter(
+            let _ = session.services.session_telemetry.counter(
                 "codex.task.compact",
                 1,
                 &[("type", "remote")],
             );
             crate::compact_remote::run_remote_compact_task(session.clone(), ctx).await
         } else {
-            let _ = session.services.otel_manager.counter(
+            let _ = session.services.session_telemetry.counter(
                 "codex.task.compact",
                 1,
                 &[("type", "local")],