fix(traces): downgrade handle_proxy body-read failure from ERROR to WARN (#1046)

JIRA: https://datadoghq.atlassian.net/browse/SLES-2729
Issue: #1000

**Overview**
When AWS Lambda freezes or terminates a function instance, the OS
immediately closes all TCP sockets. The extension's proxy HTTP handler
(handle_proxy) was mid-read on the request body — this read fails with
error reading a body from connection and was logged at ERROR level,
generating noise with no actionable signal.

This is expected Lambda lifecycle behaviour: the Lambda function
completed successfully and nothing in the extension is broken. The ERROR
level implies an extension fault, which misleads operators.

**Change**
Introduce a warn_response helper (mirroring the existing error_response
/ success_response trio) and use it for body-read failures in
handle_proxy. The log message content is unchanged — only the severity
is downgraded from ERROR to WARN.

Testing
- unit tests

Author: litianningdatadog

bottlecap/src/traces/trace_agent.rs

@@ -659,7 +659,7 @@ impl TraceAgent {
         let (parts, body) = match extract_request_body(request).await {
             Ok(r) => r,
             Err(e) => {
-                return error_response(
+                return warn_response(
                     StatusCode::INTERNAL_SERVER_ERROR,
                     format!("TRACE_AGENT | handle_proxy | Error extracting request body: {e}"),
                 );
@@ -721,6 +721,13 @@ fn error_response<E: std::fmt::Display>(status: StatusCode, error: E) -> Respons
     (status, error.to_string()).into_response()
 }
 
+/// Like [`error_response`], but logs at WARN level. Use when the failure is caused by an
+/// external event (e.g. client disconnected) rather than a bug in the extension itself.
+fn warn_response<E: std::fmt::Display>(status: StatusCode, error: E) -> Response {
+    warn!("{}", error);
+    (status, error.to_string()).into_response()
+}
+
 fn success_response(message: &str) -> Response {
     debug!("{}", message);
     (StatusCode::OK, json!({"rate_by_service": {}}).to_string()).into_response()