feat: add list_functions client APIs (#4108)

Function registration and exact lookup are exposed through the SDK, but
clients cannot discover published versions even though the server
provides `POST /v1/functions/list`.

Add Rust and Python sync/async `list_functions()` APIs that return typed
`FunctionVersion` values. The remote client requests canonical
definitions and follows opaque page tokens until the listing is
complete, including empty intermediate pages, while preserving the
server's name/version ordering. Local databases retain the existing
Function-catalog unsupported error.

The SDK consumes protocol pagination internally so callers receive the
complete catalog rather than handling server-specific page tokens.

python/src/connection.rs

@@ -629,6 +629,19 @@ impl Connection {
         })
     }
 
+    pub fn list_functions(self_: PyRef<'_, Self>) -> PyResult<Bound<'_, PyAny>> {
+        let inner = self_.get_inner()?.clone();
+        future_into_py(self_.py(), async move {
+            inner
+                .list_functions()
+                .await
+                .infer_error()?
+                .into_iter()
+                .map(|function| function.to_canonical_json().infer_error())
+                .collect::<PyResult<Vec<_>>>()
+        })
+    }
+
     pub fn drop_function(
         self_: PyRef<'_, Self>,
         name: String,