SMOODEV-2023: Rust optional tower/axum + reqwest instrumentation helpers

[brentrager]

Adds feature-gated (default-off) HTTP instrumentation to smooai-observability: tower feature → OtelTraceLayer (per-request server span on the SDK's global tracer, route-template extractor, context nesting); reqwest-middleware feature → OtelReqwestMiddleware (client spans) + instrument_client_call fallback. Deliberately uses the direct OTel API (not tower_http::TraceLayer, which needs a tracing-otel bridge the crate doesn't install). fmt+clippy(--all-features,-D warnings)+test(--all-features, 63) green; builds verified for default/tower-only/reqwest-only/all-features. Part of SMOODEV-2023 / epic 1981.

Note: W3C traceparent header injection on outbound reqwest calls is a follow-up (no propagator wired yet).

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