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/tests/telemetry.spec.ts

@@ -262,6 +262,23 @@ describe('TelemetryCoordinator lifecycle and containment', () => {
     expect(ops.every(r => !('event.seq' in r.attributes) && !('event.type' in r.attributes))).toBe(true)
   })
 
+  it('retires a disposed session: no retention, no stale shutdown marker at unload', async () => {
+    const { ctx, backend, fiber } = await setup()
+    liveSession(ctx, 'survivor')
+    // A session owned by its own fiber: disposing the fiber detaches it from
+    // the store and emits `session/disposed` — the authoritative retirement
+    // signal a long-lived telemetry backend must honor, or every closed
+    // session (and its full event log) stays strongly held for the backend's
+    // lifetime and final unload emits shutdown markers for dead sessions.
+    const owner = await ctx.plugin(Object.assign((inner: Context) => {
+      inner.sessions.create(SessionId('ephemeral'), { meta: {} })
+    }, { inject: ['sessions'] }))
+    await owner.dispose()
+    await fiber.dispose()
+    const ops = backend.records.filter(r => r.channel === 'ops')
+    expect(ops.map(r => r.attributes['session.id'])).toEqual(['survivor'])
+  })
+
   it('warns instead of throwing when backend shutdown fails', async () => {
     const backend = new FakeBackend()
     backend.shutdownError = new Error('exporter unreachable')