feat(model): add helpers to build enum from concrete values (#393)

This allows building functions like

```rust
fn stream(resp: impl Into<ServerResult>, req_id: RequestId) -> Result<Response, UpstreamError> {
	let rpc = ServerJsonRpcMessage::response(resp.into(), req_id);
```

crates/rmcp/src/model.rs

@@ -1474,6 +1474,14 @@ macro_rules! ts_union {
         pub enum $U {
             $($V($V),)*
         }
+
+        $(
+            impl From<$V> for $U {
+                fn from(value: $V) -> Self {
+                    $U::$V(value)
+                }
+            }
+        )*
     };
 }
 
@@ -1598,17 +1606,6 @@ impl TryInto<CancelledNotification> for ClientNotification {
         }
     }
 }
-impl From<CancelledNotification> for ServerNotification {
-    fn from(value: CancelledNotification) -> Self {
-        ServerNotification::CancelledNotification(value)
-    }
-}
-
-impl From<CancelledNotification> for ClientNotification {
-    fn from(value: CancelledNotification) -> Self {
-        ClientNotification::CancelledNotification(value)
-    }
-}
 
 // =============================================================================
 // TESTS