feat: migrate exchange_island CLI buy command to use centralized aitbc package HTTP client

- Replace httpx.Client with aitbc.AITBCHTTPClient in buy command
- Remove async context manager in favor of direct AITBCHTTPClient usage
- Replace status code checks with NetworkError exception handling
- Remove httpx import (no longer needed)
- Simplify error handling with separate NetworkError and generic Exception catches

cli/aitbc_cli/commands/exchange_island.py

@@ -101,16 +101,8 @@ def buy(ctx, ait_amount: float, quote_currency: str, max_price: Optional[float])
 
         # Submit transaction to blockchain
         try:
-            import httpx
-            with httpx.Client() as client:
-                response = client.post(
-                    f"{rpc_endpoint}/transaction",
-                    json=buy_order_data,
-                    timeout=10
-                )
-                
-                if response.status_code == 200:
-                    result = response.json()
+            http_client = AITBCHTTPClient(base_url=rpc_endpoint, timeout=10)
+            result = http_client.post("/transaction", json=buy_order_data)
             success(f"Buy order created successfully!")
             success(f"Order ID: {order_id}")
             success(f"Buying {ait_amount} AIT with {quote_currency}")
@@ -128,15 +120,12 @@ def buy(ctx, ait_amount: float, quote_currency: str, max_price: Optional[float])
                 "User": user_id[:16] + "...",
                 "Island": island_id[:16] + "..."
             }
-                    
             output(order_info, ctx.obj.get('output_format', 'table'))
-                else:
-                    error(f"Failed to submit transaction: {response.status_code}")
-                    if response.text:
-                        error(f"Error details: {response.text}")
+        except NetworkError as e:
+            error(f"Network error submitting transaction: {e}")
             raise click.Abort()
         except Exception as e:
-            error(f"Network error submitting transaction: {e}")
+            error(f"Error submitting transaction: {e}")
             raise click.Abort()
 
     except Exception as e: