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/pyproject.toml

@@ -13,6 +13,8 @@ dependencies = [
     "deepdiff>=8.0.1",
     "pytest-retry>=1.6.3",
     "pytest-httpserver>=1.1.0",
+    "openai>=1.0.0",
+    "anthropic>=0.66.0",
 ]
 
 [project.optional-dependencies]