Fix potential nil pointer dereference in primary key parsing

Fixed a potential panic in PostgreSQL primary key constraint parsing
by using safe type assertion with the comma-ok idiom instead of an
unsafe type assertion.

Changes:
- Replaced unsafe type assertion with checked type assertion
- Added conditional check before accessing String_.Sval
- Removed FIXME comment as issue is now resolved

This prevents potential runtime panics if key.Node is not actually
a *nodes.Node_String_ type, making the code more robust and defensive.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

internal/engine/postgresql/parse.go

@@ -418,8 +418,9 @@ func translate(node *nodes.Node) (ast.Node, error) {
 			case *nodes.Node_Constraint:
 				if item.Constraint.Contype == nodes.ConstrType_CONSTR_PRIMARY {
 					for _, key := range item.Constraint.Keys {
-						// FIXME: Possible nil pointer dereference
-						primaryKey[key.Node.(*nodes.Node_String_).String_.Sval] = true
+						if str, ok := key.Node.(*nodes.Node_String_); ok {
+							primaryKey[str.String_.Sval] = true
+						}
 					}
 				}