Improve error handling for AI provider connections

src/ai/http.ts

@@ -31,7 +31,20 @@ export async function postChatCompletion(
       signal: controller.signal,
     });
   } catch (e) {
-    throw new ProviderError(e instanceof Error ? e.message : 'Network error');
+    if (e instanceof Error) {
+      if (e.name === 'AbortError') {
+        throw new ProviderError(`Request timed out after 30s connecting to ${url}`);
+      }
+      const cause = (e as any).cause;
+      if (cause?.code === 'ECONNREFUSED') {
+        throw new ProviderError(`Connection refused to ${url}. Is the server running?`);
+      }
+      if (cause?.code === 'ENOTFOUND') {
+        throw new ProviderError(`Address not found: ${url}. Check your settings.`);
+      }
+      throw new ProviderError(`Network error: ${e.message}`);
+    }
+    throw new ProviderError('Network error');
   }
 
   if (!res.ok) {

src/test/http_error.test.ts

@@ -0,0 +1,29 @@
+import * as assert from 'assert';
+import { postChatCompletion } from '../ai/http';
+import { ProviderError } from '../ai/errors';
+
+suite('HTTP Error Handling', () => {
+    test('ECONNREFUSED returns friendly message', async () => {
+        const url = 'http://localhost:11111/v1/chat/completions'; // Port likely closed
+        try {
+            await postChatCompletion(url, undefined, { model: 'test', messages: [] });
+            assert.fail('Should have thrown');
+        } catch (e) {
+            assert.ok(e instanceof ProviderError, `Expected ProviderError but got ${e}`);
+            assert.ok(e.message.includes('Connection refused'), `Expected 'Connection refused' in '${e.message}'`);
+            assert.ok(e.message.includes(url), `Expected URL in error message '${e.message}'`);
+        }
+    });
+
+    test('ENOTFOUND returns friendly message', async () => {
+        const url = 'http://nonexistent-domain-xyz.local/v1/chat/completions';
+        try {
+            await postChatCompletion(url, undefined, { model: 'test', messages: [] });
+            assert.fail('Should have thrown');
+        } catch (e) {
+            assert.ok(e instanceof ProviderError, `Expected ProviderError but got ${e}`);
+            assert.ok(e.message.includes('Address not found'), `Expected 'Address not found' in '${e.message}'`);
+            assert.ok(e.message.includes(url), `Expected URL in error message '${e.message}'`);
+        }
+    });
+});