fix(theme): return Less_Tree_Keyword for boolean custom Less functions (#4405)

Boolean return values from `addCustomLessFunction` were wrapped in
`Less_Tree_Quoted`, producing a quoted string ('true'/'false') instead
of a Less keyword (true/false). Quoted strings do not match in mixin
guards, making it impossible to use boolean-returning functions with
`.mixin(false) {}` patterns.

Switch to `Less_Tree_Keyword` so booleans behave as proper Less keywords.
CSS custom property output is unchanged; only mixin guard matching is fixed.

Fixes #4383

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: imorland

framework/core/src/Extend/Theme.php

@@ -84,7 +84,7 @@ public function addCustomLessFunction(string $functionName, callable $callable):
             $return = $callable(...$argVals);
 
             if (is_bool($return)) {
-                return new \Less_Tree_Quoted('', $return ? 'true' : 'false');
+                return new \Less_Tree_Keyword($return ? 'true' : 'false');
             }
 
             if (is_string($return)) {

framework/core/tests/fixtures/less/custom_function.less

@@ -5,3 +5,10 @@
   --x: is-flarum("not flarum") * 10;
   --y: is-gt(1, 2);
 }
+.mixin(@val) {}
+.mixin(false) {
+  color: red;
+}
+.dummy_func_test3 {
+  .mixin(is-gt(1, 2));
+}

framework/core/tests/integration/extenders/ThemeTest.php

@@ -131,6 +131,8 @@ public function theme_extender_can_add_custom_function()
 
         $this->assertStringContainsString('.dummy_func_test{color:green}', $contents);
         $this->assertStringContainsString('.dummy_func_test2{--x:1000;--y:false}', $contents);
+        // Boolean return values must be Less keywords (not quoted strings) so that mixin guards match correctly.
+        $this->assertStringContainsString('.dummy_func_test3{color:red}', $contents);
     }
 
     /**