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/runtime_summary.rs

@@ -1,6 +1,6 @@
-use codex_otel::OtelManager;
 use codex_otel::RuntimeMetricTotals;
 use codex_otel::RuntimeMetricsSummary;
+use codex_otel::SessionTelemetry;
 use codex_otel::TelemetryAuthMode;
 use codex_otel::metrics::MetricsClient;
 use codex_otel::metrics::MetricsConfig;
@@ -20,7 +20,7 @@ fn runtime_metrics_summary_collects_tool_api_and_streaming_metrics() -> Result<(
         MetricsConfig::in_memory("test", "codex-cli", env!("CARGO_PKG_VERSION"), exporter)
             .with_runtime_reader(),
     )?;
-    let manager = OtelManager::new(
+    let manager = SessionTelemetry::new(
         ThreadId::new(),
         "gpt-5.1",
         "gpt-5.1",