fix(telemetry): review fixes — flush/shutdown ordering, session retirement, whole-exporter passthrough

Three review findings, each pinned by a red test first:

- The OTel backend retains the latest turn-boundary flush promise and
  shutdown() awaits it before provider.shutdown(): the SDK's
  concurrent-flush guard makes the shutdown-internal flush return early
  while one is in flight, silently dropping everything enqueued after
  the flush snapshot (including the coordinator's dispose-time shutdown
  markers).
- The coordinator retires sessions from the adopted set on
  session/disposed (mirroring the persistence coordinator): a long-lived
  backend no longer retains closed sessions and their event logs, and
  final unload no longer stamps shutdown markers for dead sessions.
- The exporter config passes through whole to OTLPLogExporter and its
  type/JSDoc now advertise the full OTLPExporterNodeConfigBase shape:
  rebuilding url/headers only silently dropped documented SDK options
  (timeoutMillis, compression, keepAlive, ...).

pnpm-lock.yaml

@@ -3567,6 +3567,9 @@ importers:
       '@opentelemetry/exporter-logs-otlp-http':
         specifier: ^0.220.0
         version: 0.220.0(@opentelemetry/api@1.9.1)
+      '@opentelemetry/otlp-exporter-base':
+        specifier: ^0.220.0
+        version: 0.220.0(@opentelemetry/api@1.9.1)
       '@opentelemetry/resources':
         specifier: ^2.9.0
         version: 2.10.0(@opentelemetry/api@1.9.1)