feat: expand PostgreSQL TABLE and VALUES query coverage

Parse standalone TABLE and VALUES expressions through the compound-query
parser, and allow them to participate in mixed UNION, INTERSECT and EXCEPT
queries. Preserve explicit parentheses and local ORDER BY/LIMIT clauses so
emitted SQL keeps the original grouping and set-operation precedence.

Add operand completeness checks for VALUES without removing the permissive
operator handling used by existing callers. Teach the planner to unwrap
supported grouped queries and reject operands it cannot execute, rather
than constructing an incomplete plan. This adds parsing and reconstruction;
it does not implement local TABLE or VALUES execution.

Cover AST contents, nested grouping, mixed operands, quoted relations,
round trips, incomplete values and planner rejection. Verified the staged
source snapshot independently: all 45 compound-query and planner tests
passed. PostgreSQL compatibility snapshots are refreshed in a later commit
of this series after the remaining grammar additions.

Author: renecannao

include/sql_engine/plan_builder.h

@@ -606,7 +606,11 @@ class PlanBuilder {
         const sql_parser::AstNode* set_op_node = find_child(compound_ast, sql_parser::NodeType::NODE_SET_OPERATION);
         if (set_op_node) {
             current = build_set_op(set_op_node);
+        } else if (compound_ast->first_child) {
+            // Parenthesized operands preserve their own ORDER BY/LIMIT scope.
+            current = build(compound_ast->first_child);
         }
+        if (!current) return nullptr;
 
         // Trailing ORDER BY
         const sql_parser::AstNode* order_by = find_child(compound_ast, sql_parser::NodeType::NODE_ORDER_BY_CLAUSE);
@@ -679,6 +683,7 @@ class PlanBuilder {
             }
         }
 
+        if (!node->left || !node->right) return nullptr;
         return node;
     }
 };

include/sql_parser/common.h

@@ -58,6 +58,10 @@ inline int ci_cmp(const char* a, uint32_t alen, const char* b, uint8_t blen) {
 
 // -- Flags for NODE_SET_OPERATION --
 static constexpr uint16_t FLAG_SET_OP_ALL = 0x01;
+// Parenthesized query expressions retain grouping during SQL emission.
+static constexpr uint16_t FLAG_QUERY_PARENTHESIZED = 0x01;
+static constexpr uint16_t FLAG_TABLE_ONLY = 0x01;
+static constexpr uint16_t FLAG_TABLE_INHERIT = 0x02;
 
 // -- Flags for NODE_IDENTIFIER / NODE_COLUMN_REF --
 // Set when the identifier was source-delimited (backtick `name` for MySQL,
@@ -237,6 +241,7 @@ enum class NodeType : uint16_t {
     NODE_USER_VARIABLE,
     NODE_LITERAL_HEX,
     NODE_LITERAL_BIT,
+    NODE_TABLE_QUERY,           // PostgreSQL TABLE [ONLY] relation [*]
 };
 
 } // namespace sql_parser