fix: support ConstExpr with functions defined via expr.Function

ConstExpr panics when the named function was defined using
expr.Function() instead of being a method on the environment
struct. This is because ConstExpr only looks in the environment
object via runtime.Fetch, not in the Functions table.

Check c.Functions first and extract the appropriate function
implementation (Func, Fast, or Safe) before falling back to
the environment object lookup.

Fixes #809

Author: Yanhu007

conf/config.go

@@ -74,6 +74,22 @@ func (c *Config) WithEnv(env any) {
 }
 
 func (c *Config) ConstExpr(name string) {
+	// Check if the function is defined via expr.Function first.
+	if fn, ok := c.Functions[name]; ok {
+		if fn.Func != nil {
+			c.ConstFns[name] = reflect.ValueOf(fn.Func)
+			return
+		}
+		if fn.Fast != nil {
+			c.ConstFns[name] = reflect.ValueOf(fn.Fast)
+			return
+		}
+		if fn.Safe != nil {
+			c.ConstFns[name] = reflect.ValueOf(fn.Safe)
+			return
+		}
+		panic(fmt.Errorf("const expression %q must have a function implementation", name))
+	}
 	if c.EnvObject == nil {
 		panic("no environment is specified for ConstExpr()")
 	}