Otel: dedupe LLM span error/status + traceparent helpers (#130 follow-up)

Addresses the P2 refactoring follow-ups parked on issue #130:

- otel::with_traceparent(builder, &SpanContext) — one shared W3C
  traceparent injector. Replaces the 4-line inline header loop in
  llamacpp (3x) and openrouter (3x) and removes ollama's private
  inject_traceparent method (4 call sites now use the shared helper).
  Takes &SpanContext (not &dyn Span) because the Span trait is not dyn
  compatible in opentelemetry 0.31.

- otel::record_http_failure(span, status, message) and
  otel::set_http_status(span, status) — collapse the repeated
  'set http.status_code attribute + set_status(Status::error)'
  boilerplate across all three clients (~10 sites).

Net -45 lines. The remaining if-let-Some(span.as_mut()) guards (token
attributes, parse-failure status) stay inline rather than growing a
SpanHandle wrapper type for a pattern that's already readable.

src/otel.rs

@@ -2,7 +2,9 @@ use actix_web::HttpRequest;
 use actix_web::http::header::HeaderMap;
 use opentelemetry::global::{BoxedSpan, BoxedTracer};
 use opentelemetry::propagation::TextMapPropagator;
-use opentelemetry::trace::{Span, SpanBuilder, SpanKind, Status, TraceContextExt, Tracer};
+use opentelemetry::trace::{
+    Span, SpanBuilder, SpanContext, SpanKind, Status, TraceContextExt, Tracer,
+};
 use opentelemetry::{Context, KeyValue, global};
 use opentelemetry_appender_log::OpenTelemetryLogBridge;
 use opentelemetry_otlp::WithExportConfig;
@@ -117,6 +119,38 @@ pub fn traceparent_headers(context: &Context) -> Vec<(String, String)> {
     headers.into_iter().filter(|(_, v)| !v.is_empty()).collect()
 }
 
+/// Attach the W3C traceparent header to an outgoing request so a downstream
+/// service parents its own spans onto this trace. No-op when the span context
+/// is not valid — e.g. debug builds, where the global tracer is a no-op and an
+/// all-zero traceparent would produce garbage parent spans.
+pub fn with_traceparent(
+    builder: reqwest::RequestBuilder,
+    span_context: &SpanContext,
+) -> reqwest::RequestBuilder {
+    let mut b = builder;
+    for (k, v) in
+        traceparent_headers(&Context::new().with_remote_span_context(span_context.clone()))
+    {
+        b = b.header(k, v);
+    }
+    b
+}
+
+/// Record the HTTP status code on the span (success or failure path).
+pub fn set_http_status(span: Option<&mut BoxedSpan>, status: u16) {
+    if let Some(s) = span {
+        s.set_attribute(KeyValue::new("http.status_code", status as i64));
+    }
+}
+
+/// Record an HTTP failure on the span: status code plus an error status.
+pub fn record_http_failure(span: Option<&mut BoxedSpan>, status: u16, message: &str) {
+    if let Some(s) = span {
+        s.set_attribute(KeyValue::new("http.status_code", status as i64));
+        s.set_status(Status::error(message.to_string()));
+    }
+}
+
 pub fn trace_db_call<F, O>(
     context: &Context,
     query_type: &str,