SMOODEV-2026: .NET observability transport via SmooAI.Fetch

[brentrager]

Webhook transport now goes through SmooAI.Fetch (timeout + exponential-backoff retries + circuit breaker), wire bytes unchanged (pre-serialized StringContent via SendAsync). ClientOptions.HttpClient escape hatch preserved. No double-retry (smooai-fetch retries within a flush; batch-requeue only after). OTLP deferred — SmooAI.Fetch isn't HttpClient/DelegatingHandler-shaped so the OTLP HttpClientFactory can't bind it without reimplementing the protobuf POST; clean enabler would be a SmooAI.Fetch.CreateMessageHandler(). build+test(69)+format green. SMOODEV-2026 / epic 1981.

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