fix(proxy): delete prepared statements when discarding (#11165)

Fixes https://github.com/neondatabase/serverless/issues/144

When tables have enums, we need to perform type queries for that data.
We cache these query statements for performance reasons. In Neon RLS, we
run "discard all" for security reasons, which discards all the
statements. When we need to type check again, the statements are no
longer valid.

This fixes it to discard the statements as well.

I've also added some new logs and error types to monitor this. Currently
we don't see the prepared statement errors in our logs.

libs/proxy/tokio-postgres2/src/client.rs

@@ -284,6 +284,18 @@ impl Client {
         simple_query::batch_execute(self.inner(), query).await
     }
 
+    pub async fn discard_all(&self) -> Result<ReadyForQueryStatus, Error> {
+        // clear the prepared statements that are about to be nuked from the postgres session
+        {
+            let mut typeinfo = self.inner.cached_typeinfo.lock();
+            typeinfo.typeinfo = None;
+            typeinfo.typeinfo_composite = None;
+            typeinfo.typeinfo_enum = None;
+        }
+
+        self.batch_execute("discard all").await
+    }
+
     /// Begins a new database transaction.
     ///
     /// The transaction will roll back by default - use the `commit` method to commit it.