test(e2e): consolidate selector tests, add round-robin/fifo/backup failover

Merge parallel_selector_test.go into selector_test.go and add e2e coverage
for the round-robin + fail filter, fifo sticky fallback, and backup filter
strategies. Each test drives requests through a hop with one dead node and
asserts the selector marks and skips it so traffic converges on the live node.

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

tests/e2e/testdata/selector/parallel.yaml

@@ -0,0 +1,30 @@
+services:
+- name: proxy
+  addr: :8080
+  handler:
+    type: http
+    chain: my-chain
+  listener:
+    type: tcp
+
+- name: dummy-1
+  addr: :18081
+  handler:
+    type: http
+
+chains:
+- name: my-chain
+  hops:
+  - name: hop-1
+    selector:
+      strategy: parallel
+    nodes:
+    - name: node-1
+      addr: 127.0.0.1:18081
+      connector:
+        type: http
+    # non existed node
+    - name: node-2
+      addr: 127.0.0.1:18082
+      connector:
+        type: http