fix: skip multi-statement SET validation in session mode

pgdog/src/frontend/router/parser/context.rs

@@ -149,4 +149,14 @@ impl<'a> QueryParserContext<'a> {
     pub(super) fn expanded_explain(&self) -> bool {
         self.expanded_explain
     }
+
+    /// Are we running in session mode?
+    ///
+    /// In session mode, queries are forwarded to the server without
+    /// parsing or validation beyond what's required for routing.
+    pub(super) fn is_session_mode(&self) -> bool {
+        self.router_context
+            .cluster
+            .pooler_mode() == crate::config::PoolerMode::Session
+    }
 }

pgdog/src/frontend/router/parser/query/set.rs

@@ -60,11 +60,19 @@ impl QueryParser {
     /// - All SETs → returns `Ok(Some(Command::Set { .. }))`
     /// - No SETs → returns `Ok(None)`, caller falls through to default parsing
     /// - Mix of SET + non-SET → returns `Err(MultiStatementMixedSet)`
+    ///
+    /// In session mode, the entire check is bypassed (`Ok(None)`) so that
+    /// all multi-statement queries are forwarded to the server verbatim.
     pub(super) fn try_multi_set(
         &mut self,
         stmts: &[RawStmt],
         context: &QueryParserContext,
     ) -> Result<Option<Command>, Error> {
+        // In session mode, pass through without validation — the server
+        // owns the session and can handle mixed SET + other statements.
+        if context.is_session_mode() {
+            return Ok(None);
+        }
         let mut params = Vec::with_capacity(stmts.len());
         let mut has_set = false;
         let mut has_other = false;