api: proper error messages for connection failures and HTTP errors

- Connection errors now show cause (refused/timeout/request error),
  URL, and the underlying error without redundant URL repetition
- HTTP errors show status code, URL, and up to 1000 chars of body
- Unparseable SSE events logged with content preview instead of
  silently dropped — may contain error info from vllm/server
- Stream errors already had good context (kept as-is)

You can't debug what you can't see.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poc-agent/src/api/mod.rs

@@ -138,7 +138,18 @@ pub(crate) async fn send_and_check(
         .json(body)
         .send()
         .await
-        .map_err(|e| anyhow::anyhow!("Failed to send request to API: {}", e))?;
+        .map_err(|e| {
+            let cause = if e.is_connect() {
+                "connection refused"
+            } else if e.is_timeout() {
+                "request timed out"
+            } else if e.is_request() {
+                "request error"
+            } else {
+                "unknown"
+            };
+            anyhow::anyhow!("{} ({}): {}", cause, url, e.without_url())
+        })?;
 
     let status = response.status();
     let elapsed = start.elapsed();
@@ -164,12 +175,13 @@ pub(crate) async fn send_and_check(
     if !status.is_success() {
         let body = response.text().await.unwrap_or_default();
         let _ = ui_tx.send(UiMessage::Debug(format!(
-            "API error {} after {:.1}s: {}",
+            "HTTP {} after {:.1}s ({}): {}",
             status,
             elapsed.as_secs_f64(),
-            &body[..body.len().min(300)]
+            url,
+            &body[..body.len().min(500)]
         )));
-        anyhow::bail!("API error {}: {}", status, &body[..body.len().min(500)]);
+        anyhow::bail!("HTTP {} ({}): {}", status, url, &body[..body.len().min(1000)]);
     }
 
     if debug {

poc-agent/src/api/openai.rs

@@ -79,9 +79,17 @@ pub async fn stream(
             anyhow::bail!("API error in stream: {} {}", err_msg, raw);
         }
 
-        let chunk: ChatCompletionChunk = match serde_json::from_value(event) {
+        let chunk: ChatCompletionChunk = match serde_json::from_value(event.clone()) {
             Ok(c) => c,
-            Err(_) => continue,
+            Err(e) => {
+                // Log unparseable events — they may contain error info
+                let preview = event.to_string();
+                let _ = ui_tx.send(UiMessage::Debug(format!(
+                    "unparseable SSE event ({}): {}",
+                    e, &preview[..preview.len().min(300)]
+                )));
+                continue;
+            }
         };
 
         if chunk.usage.is_some() {