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/state/src/runtime/threads.rs

@@ -447,7 +447,7 @@ ON CONFLICT(thread_id, position) DO NOTHING
         &self,
         builder: &ThreadMetadataBuilder,
         items: &[RolloutItem],
-        otel: Option<&OtelManager>,
+        otel: Option<&SessionTelemetry>,
         new_thread_memory_mode: Option<&str>,
         updated_at_override: Option<DateTime<Utc>>,
     ) -> anyhow::Result<()> {