test: regression test for @container mixin parameter variable resolution (#4420)

* Initial plan

* Initial plan: add regression test for container mixin parameters issue

Co-authored-by: matthew-dean <414752+matthew-dean@users.noreply.github.com>

* test: add regression test for container mixin parameters issue (#4420)

Co-authored-by: matthew-dean <414752+matthew-dean@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: matthew-dean <414752+matthew-dean@users.noreply.github.com>
Co-authored-by: Matthew Dean <matthew-dean@users.noreply.github.com>

Author: Copilot

packages/test-data/tests-unit/container/container.css

@@ -273,3 +273,21 @@
     font-size: 75%;
   }
 }
+.mixin-container-test {
+  color: red;
+}
+@container name (width < 125px) {
+  .mixin-container-test {
+    display: none;
+  }
+}
+@container sidebar (width < 500px) {
+  .sidebar-test {
+    display: none;
+  }
+}
+@container header (width < 800px) {
+  .header-test {
+    display: none;
+  }
+}

packages/test-data/tests-unit/container/container.less

@@ -326,3 +326,27 @@
       font-size: 75%;
     }
 }
+
+// Regression test: mixin with variable container name and variable query condition
+// Issue: mixins with parameters using @container @name (condition < @var) failed
+// with "variable @bp is undefined" error in older versions
+@issue-width: 125px;
+
+.container-query-mixin(@container-name; @bp) {
+    @container @container-name (width < @bp) {
+        display: none;
+    }
+}
+
+.mixin-container-test {
+    .container-query-mixin(name, @issue-width);
+    color: red;
+}
+
+// Verify multiple calls with different params produce correct output
+.sidebar-test {
+    .container-query-mixin(sidebar, 500px);
+}
+.header-test {
+    .container-query-mixin(header, 800px);
+}