feat(python): support exist_ok when creating secrets (#4978)

Add `exist_ok=False` to synchronous and asynchronous Python
`create_secret`. Passing `True` uses `CREATE SECRET IF NOT EXISTS`,
preserving an existing credential. The default continues to fail on an
existing name; rotation stays explicit through `alter_secret`.

```python
db.create_secret("api-key", value, exist_ok=True)
await async_db.create_secret("api-key", value, exist_ok=True)
```

The Rust API gains an awaitable builder with `.exist_ok(true)` while
existing `create_secret(...).await` calls keep working. Includes Python
SQL round-trip coverage and Rust coverage for direct and deferred SQL
responses.

python/src/connection.rs

@@ -1036,18 +1036,20 @@ impl Connection {
         })
     }
 
-    #[pyo3(signature = (name, value, namespace_path=None))]
+    #[pyo3(signature = (name, value, namespace_path=None, *, exist_ok=false))]
     pub fn create_secret(
         self_: PyRef<'_, Self>,
         name: String,
         value: String,
         namespace_path: Option<Vec<String>>,
+        exist_ok: bool,
     ) -> PyResult<Bound<'_, PyAny>> {
         let inner = self_.get_inner()?.clone();
         let namespace_path = namespace_path.unwrap_or_default();
         future_into_py(self_.py(), async move {
             inner
                 .create_secret(name, value, &namespace_path)
+                .exist_ok(exist_ok)
                 .await
                 .infer_error()
         })