refactor(telemetry): remove the OTel backend's flush forwarding

Three review rounds each found a new silent-loss path in the same
wrapper state (dispose racing an in-flight flush, overlapping hints
displacing the retained promise, the provider's fixed 30s flush timeout
rejecting while the processor still drains). Every path exists only
because forwarding the seam's turn-boundary hint to forceFlush() made
this backend the process's second flusher against undocumented SDK
internals from the upstream experimental tree.

The backend now implements no flush(): the batch processor is the only
flusher, its scheduledDelayMillis (already deployment-tunable through
the processor passthrough) governs export cadence, and shutdown()'s
drain is complete by construction. The two race-pin tests collapse into
one dispose-during-in-flight-batch drain pin; the seam's optional
flush() contract now tells implementers they own the concurrent-flush/
shutdown interaction. Removal rationale and the reinstatement trigger
(a stated turn-boundary latency requirement scheduledDelayMillis cannot
meet — and then via the processor's own forceFlush(), never the
provider's timeout-wrapped one) are recorded in the revival Agent Note,
both languages.

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

@@ -103,7 +103,12 @@ export interface TelemetryBackend {
    * Optional hint that a natural boundary (turn end) passed — a backend may
    * forward it to its SDK's flush so records land at turn boundaries. Called
    * fire-and-forget; implementations must not block and must not throw
-   * meaningfully (the coordinator contains exceptions).
+   * meaningfully (the coordinator contains exceptions). Most backends should
+   * leave this unimplemented and let their SDK's own batching cadence govern
+   * export timing: a backend that does implement it owns the interaction
+   * between its concurrent flushes and {@link shutdown}'s drain (the OTel
+   * backend removed its implementation for exactly that hazard — see the
+   * revival Agent Note).
    */
   flush?(): void
   /**