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/otel_export_routing_policy.rs

@@ -1,4 +1,4 @@
-use codex_otel::OtelManager;
+use codex_otel::SessionTelemetry;
 use codex_otel::TelemetryAuthMode;
 use codex_otel::otel_provider::OtelProvider;
 use opentelemetry::KeyValue;
@@ -103,7 +103,7 @@ fn otel_export_routing_policy_routes_user_prompt_log_and_trace_events() {
 
     tracing::subscriber::with_default(subscriber, || {
         tracing::callsite::rebuild_interest_cache();
-        let manager = OtelManager::new(
+        let manager = SessionTelemetry::new(
             ThreadId::new(),
             "gpt-5.1",
             "gpt-5.1",
@@ -212,7 +212,7 @@ fn otel_export_routing_policy_routes_tool_result_log_and_trace_events() {
 
     tracing::subscriber::with_default(subscriber, || {
         tracing::callsite::rebuild_interest_cache();
-        let manager = OtelManager::new(
+        let manager = SessionTelemetry::new(
             ThreadId::new(),
             "gpt-5.1",
             "gpt-5.1",