Add mock-based E2E tests and gate live tests to main/nightly

Introduce a new mock-based E2E test suite that uses pytest_httpserver to
simulate LLM provider responses, eliminating the need for real API keys
on PR builds. The mock tests cover model alias routing, protocol
transformation (OpenAI↔Anthropic), Responses API passthrough/translation,
streaming, tool calls, thinking mode, and multi-turn state management.

CI changes:
- Add mock-e2e-tests job (zero secrets, runs on every PR)
- Gate all live E2E jobs to main pushes + nightly schedule
- Scope secrets to only the keys each job actually needs
- Add daily cron schedule for full live test coverage

Also relaxes exact-match assertions in live e2e tests to structural
checks (non-null, non-empty) since LLM output is non-deterministic.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/archgw/docker-compose.mock.yaml

@@ -0,0 +1,13 @@
+services:
+  plano:
+    image: katanemo/plano:latest
+    ports:
+      - "12000:12000"
+    volumes:
+      - ./config_mock_llm.yaml:/app/plano_config.yaml
+      - /etc/ssl/cert.pem:/etc/ssl/cert.pem
+    extra_hosts:
+      - "host.docker.internal:host-gateway"
+    environment:
+      - OPENAI_API_KEY=mock-key
+      - ANTHROPIC_API_KEY=mock-key