Add a conventional per-request OTel server span using the standard
tracing-actix-web middleware (TracingLogger) as the outermost middleware,
so the whole request lives under one root span and an incoming W3C
traceparent is extracted onto it via the global text-map propagator
(installed in #138).
extract_context_from_request now sources the operation span's parent from
that root span (tracing::Span::current().context()) instead of re-parsing
the header, so per-handler operation spans nest under the root span as
one trace tree rather than duplicating it as a sibling. Header parsing
remains as a fallback for handlers invoked without the middleware (tests).
The literal per-handler span boilerplate is intentionally NOT removed here;
it becomes #[instrument] in the LLM (#140) and DAO (#141) migrations, where
the &Context threading disappears entirely.
tests/otel_traceparent.rs pins the behavior end-to-end: an incoming
traceparent's trace_id propagates to the request, the root span is parented
on the caller's (remote) span, and the operation span nests under the root
span rather than alongside it.
Cameron Cordes
7a59afc62b