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, ...).

docs/config-catalog.md

@@ -1059,12 +1059,15 @@ Requires: `sessions`
  * must fail at plugin load, not at first export.
  */
 export interface Config {
-  /** Passed verbatim to the SDK's OTLP/HTTP log exporter. */
-  exporter?: {
+  /**
+   * Passed verbatim to the SDK's OTLP/HTTP log exporter — the complete
+   * `OTLPExporterNodeConfigBase` shape (`headers`, `timeoutMillis`,
+   * `compression`, `keepAlive`, …), owned and documented by the SDK. `url`
+   * is the one field this package requires and validates itself.
+   */
+  exporter?: OTLPExporterNodeConfigBase & {
     /** Full logs endpoint (e.g. `https://collector.example.com/v1/logs`). Required; validated at plugin load. */
     url?: string
-    /** Extra request headers (auth etc.); owned and sent by the SDK exporter. */
-    headers?: Record<string, string>
   }
   /**
    * Passed verbatim to `BatchLogRecordProcessor` (minus the exporter slot,
@@ -1074,9 +1077,9 @@ export interface Config {
 }
 ```
 
-Depends on: `BatchLogRecordProcessorOptions` (`@opentelemetry/sdk-logs`)
+Depends on: `BatchLogRecordProcessorOptions` (`@opentelemetry/sdk-logs`) · `OTLPExporterNodeConfigBase` (`@opentelemetry/otlp-exporter-base`)
 
-Source: [`packages/telemetry/session-telemetry-otel/src/index.ts:39`](../packages/telemetry/session-telemetry-otel/src/index.ts)
+Source: [`packages/telemetry/session-telemetry-otel/src/index.ts:40`](../packages/telemetry/session-telemetry-otel/src/index.ts)
 
 ## `@deepseek-ai/dsh-session-title`