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, ...).

packages/telemetry/session-telemetry/src/index.ts

@@ -108,9 +108,13 @@ export interface TelemetryBackend {
   flush?(): void
   /**
    * Forward the fiber's disposal to the SDK: flush whatever is queued and
-   * reach quiescence, per the SDK's own shutdown contract. Awaited by the
-   * coordinator's dispose; a rejection is logged as a warning and never
-   * fails application teardown.
+   * reach quiescence, per the SDK's own shutdown contract. Everything
+   * emitted before this call must still be delivered — including records
+   * enqueued while a {@link flush} hint is in flight, so a backend whose SDK
+   * guards against concurrent flushes orders behind the outstanding one (the
+   * coordinator emits its dispose-time `shutdown` markers immediately before
+   * calling this). Awaited by the coordinator's dispose; a rejection is
+   * logged as a warning and never fails application teardown.
    * @returns resolves when the backend's pipeline has quiesced.
    */
   shutdown(): Promise<void>