[nexus] add test for OT_MLE_LONG_ROUTES feature (#12956)

This commit adds a new Nexus test to verify the functionality of the
MLE long routes experimental feature, which allows path costs to
exceed the standard limit of 15.

The new test `TestLongRoutes` in `test_long_routes.cpp` forms a
topology consisting of a leader and a chain of 25 routers. It then
validates that the path cost from the last router in the chain to the
leader is correctly reported as 25 using `GetPathCostToLeader()`.

Supporting changes include:
- Updating `build.sh` to support a `long_routes` build target that
  enables `OT_MLE_LONG_ROUTES`.
- Adding the `long_routes` test to `CMakeLists.txt` with the
  appropriate labels.
- Introducing a new GitHub workflow job `nexus-long-routes-tests` in
  `nexus.yml` to automate the execution of this test.

.github/workflows/nexus.yml

@@ -107,6 +107,35 @@ jobs:
       run: |
         cd build/nexus && ctest -L trel --output-on-failure
 
+  nexus-long-routes-tests:
+    name: nexus-long-routes-tests
+    runs-on: ubuntu-24.04
+    steps:
+    - name: Harden Runner
+      uses: step-security/harden-runner@5ef0c079ce82195b2a36a210272d6b661572d83e # v2.14.2
+      with:
+        egress-policy: audit # TODO: change to 'egress-policy: block' after couple of runs
+
+    - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
+      with:
+        submodules: recursive
+
+    - name: Bootstrap
+      env:
+        PR_BODY: "${{ github.event.pull_request.body }}"
+      run: |
+        sudo apt-get update
+        sudo apt-get --no-install-recommends install -y ninja-build lcov
+
+    - name: Build Nexus
+      run: |
+        mkdir -p build/nexus
+        top_builddir=build/nexus ./tests/nexus/build.sh long_routes
+
+    - name: Run LONG_ROUTES Tests
+      run: |
+        cd build/nexus && ctest -L long_routes --output-on-failure
+
   nexus-grpc-tests:
     name: nexus-grpc-tests
     runs-on: ubuntu-24.04