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/backup.yaml

@@ -0,0 +1,37 @@
+services:
+- name: proxy
+  addr: :8080
+  handler:
+    type: http
+    chain: my-chain
+  listener:
+    type: tcp
+
+# Live relay: forwards the request to the echo server (curl target).
+- name: relay
+  addr: 127.0.0.1:18081
+  handler:
+    type: http
+  listener:
+    type: tcp
+
+chains:
+- name: my-chain
+  hops:
+  - name: hop-1
+    selector:
+      strategy: round
+      maxFails: 1
+    nodes:
+    # primary: dead, non-backup.
+    - name: node-1
+      addr: 127.0.0.1:18082
+      connector:
+        type: http
+    # backup: only used once all primary nodes are dead.
+    - name: node-2
+      addr: 127.0.0.1:18081
+      connector:
+        type: http
+      metadata:
+        backup: true

tests/e2e/testdata/selector/fifo.yaml

@@ -0,0 +1,34 @@
+services:
+- name: proxy
+  addr: :8080
+  handler:
+    type: http
+    chain: my-chain
+  listener:
+    type: tcp
+
+# Live relay: forwards the request to the echo server (curl target).
+- name: relay
+  addr: 127.0.0.1:18081
+  handler:
+    type: http
+  listener:
+    type: tcp
+
+chains:
+- name: my-chain
+  hops:
+  - name: hop-1
+    selector:
+      strategy: fifo
+      maxFails: 1
+    nodes:
+    # primary: dead, so fifo must fall through to the secondary node.
+    - name: node-1
+      addr: 127.0.0.1:18082
+      connector:
+        type: http
+    - name: node-2
+      addr: 127.0.0.1:18081
+      connector:
+        type: http

tests/e2e/testdata/selector/roundrobin.yaml

@@ -0,0 +1,34 @@
+services:
+- name: proxy
+  addr: :8080
+  handler:
+    type: http
+    chain: my-chain
+  listener:
+    type: tcp
+
+# Live relay: forwards the request to the echo server (curl target).
+- name: relay
+  addr: 127.0.0.1:18081
+  handler:
+    type: http
+  listener:
+    type: tcp
+
+chains:
+- name: my-chain
+  hops:
+  - name: hop-1
+    selector:
+      strategy: round
+      maxFails: 1
+    nodes:
+    - name: node-1
+      addr: 127.0.0.1:18081
+      connector:
+        type: http
+    # dead node: never listens; must be marked and skipped by the selector.
+    - name: node-2
+      addr: 127.0.0.1:18082
+      connector:
+        type: http