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/otel/tests/suite/snapshot.rs

@@ -1,6 +1,6 @@
 use crate::harness::attributes_to_map;
 use crate::harness::find_metric;
-use codex_otel::OtelManager;
+use codex_otel::SessionTelemetry;
 use codex_otel::TelemetryAuthMode;
 use codex_otel::metrics::MetricsClient;
 use codex_otel::metrics::MetricsConfig;
@@ -69,7 +69,7 @@ fn manager_snapshot_metrics_collects_without_shutdown() -> Result<()> {
         .with_tag("service", "codex-cli")?
         .with_runtime_reader();
     let metrics = MetricsClient::new(config)?;
-    let manager = OtelManager::new(
+    let manager = SessionTelemetry::new(
         ThreadId::new(),
         "gpt-5.1",
         "gpt-5.1",