http: add HttpResponse::bytes() for binary downloads

Mirror of text(), but returns raw Bytes without lossy UTF-8 conversion.
Needed by the Telegram channel to fetch photo files.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/api/http.rs

@@ -154,6 +154,14 @@ impl HttpResponse {
         Ok(String::from_utf8_lossy(&bytes).into_owned())
     }
 
+    /// Read the entire body as raw bytes (for binary downloads).
+    pub async fn bytes(self) -> Result<Bytes> {
+        let bytes = self.body.collect().await
+            .context("reading response body")?
+            .to_bytes();
+        Ok(bytes)
+    }
+
     /// Read the entire body and deserialize as JSON.
     pub async fn json<T: serde::de::DeserializeOwned>(self) -> Result<T> {
         let bytes = self.body.collect().await