feat（ui）：Update API connection failed error message (#567)

## 📝 Pull Request Template

### 1. Related Issue
Closes # (issue number)

### 2. Type of Change (select one)
other

### 3. Description
updated the error message displayed when an API connection fails to be
more descriptive and helpful to users.

### 4. Testing
- [x] I have tested this locally.
- [x] I have updated or added relevant tests.

### 5. Checklist
- [x] I have read the [Code of Conduct](./CODE_OF_CONDUCT.md)
- [x] I have followed the [Contributing Guidelines](./CONTRIBUTING.md)
- [x] My changes follow the project's coding style

---------

Co-authored-by: MXD66 <239960619+MXD66@users.noreply.github.com>

Author: MXD66

frontend/src/components/valuecell/form/exchange-form.tsx

@@ -128,7 +128,9 @@ export const ExchangeForm = withForm({
       } catch (_error) {
         setTestStatus({
           success: false,
-          message: "Failed, please check your API key",
+          message:
+            "Connection failed. Please check your API Key, Secret Key, or " +
+            "Passphrase.",
         });
       }
     };

python/valuecell/server/api/routers/strategy_agent.py

@@ -282,7 +282,11 @@ async def test_exchange_connection(request: ExchangeConfig):
                     # But SuccessResponse implies 200.
                     # If I raise HTTPException it shows as error.
                     raise HTTPException(
-                        status_code=400, detail="Failed, please check your API key"
+                        status_code=400,
+                        detail=(
+                            "Connection failed. Please check your API Key, "
+                            "Secret Key, or Passphrase."
+                        ),
                     )
             finally:
                 await gateway.close()
@@ -291,7 +295,11 @@ async def test_exchange_connection(request: ExchangeConfig):
             # If create_ccxt_gateway fails or other error, avoid logging sensitive info
             logger.warning("Connection test failed")
             raise HTTPException(
-                status_code=400, detail="Failed, please check your API key"
+                status_code=400,
+                detail=(
+                    "Connection failed. Please check your API Key, "
+                    "Secret Key, or Passphrase."
+                ),
             )
 
     @router.delete("/delete")