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.
