Include full error chain in procedure HTTP request errors (#4610)

Summary
When an HTTP request fails inside a procedure, the error message only
shows something like "error sending request for url (https://...)" with
no details about what actually went wrong.

This change walks the full error chain so the message includes the root
cause — things like DNS failures, connection refused, timeouts, etc.

- Before: error sending request for url (https://httpbin.org/get)
- After: error sending request for url (https://httpbin.org/get): error
trying to connect: dns error: failed to lookup address information:
   

  Fixes #4608

  Test plan

- Trigger an HTTP failure in a procedure and check that the error
message now shows the actual reason for the failure
- Existing tests should still pass since this only changes error
formatting

Co-authored-by: Phoebe Goldman <phoebe@clockworklabs.io>

Author: Shiven0504

crates/core/src/host/instance_env.rs

@@ -857,8 +857,18 @@ impl InstanceEnv {
             err
         }
 
-        fn http_error<E: ToString>(err: E) -> NodesError {
-            NodesError::HttpError(err.to_string())
+        fn http_error<E: std::error::Error>(err: E) -> NodesError {
+            // Include the full error chain, not just the top-level message.
+            // `reqwest::Error` wraps underlying causes (DNS failure, connection refused,
+            // timeout, TLS errors, etc.) which are essential for debugging.
+            use std::fmt::Write;
+            let mut message = err.to_string();
+            let mut source = err.source();
+            while let Some(cause) = source {
+                write!(message, ": {cause}").unwrap();
+                source = cause.source();
+            }
+            NodesError::HttpError(message)
         }
 
         // Then convert the request into an `http::Request`, a semi-standard "lingua franca" type in the Rust ecosystem,
@@ -888,7 +898,7 @@ impl InstanceEnv {
 
         // Check if we have a blocked IP address, since IP literals bypass DNS resolution.
         if is_blocked_ip_literal(reqwest.url()) {
-            return Err(http_error(BLOCKED_HTTP_ADDRESS_ERROR));
+            return Err(NodesError::HttpError(BLOCKED_HTTP_ADDRESS_ERROR.to_string()));
         }
 
         let redirect_policy = reqwest::redirect::Policy::custom(|attempt| {