fix(tests): resolve 12 CI failures + 10 errors across 6 root causes (#11040)

Group A (3 tests): 'No LLM provider configured' RuntimeError
- test_user_message_surrogates_sanitized, test_counters_initialized_in_init,
  test_openai_prompt_tokens_unchanged
- Root cause: AIAgent.__init__ now requires base_url alongside api_key to
  skip resolve_provider_client() (which returns None when API keys are
  blanked in CI). Added base_url='http://localhost:1234/v1' to test
  agent construction.

Group B (5 tests): Discord slash command auto-registration
- test_auto_registers_missing_gateway_commands, test_auto_registered_command_*,
  test_register_skill_group_*
- Root cause: xdist workers that loaded a discord mock WITHOUT
  app_commands.Command/Group caused _register_slash_commands() to fail
  silently. Added comprehensive shared discord mock in
  tests/gateway/conftest.py (same pattern as existing telegram mock).

Group C (5 errors): Discord reply mode 'NoneType has no DMChannel'
- All TestReplyToText tests
- Root cause: FakeDMChannel was not a subclass of real discord.DMChannel,
  so isinstance() checks in _handle_message failed when running in full
  suite (real discord installed). Made FakeDMChannel inherit from
  discord.DMChannel when available. Removed fragile monkeypatch approach.

Group D (2 tests): detect_provider_for_model wrong provider
- test_openrouter_slug_match (got 'ai-gateway'), test_bare_name_gets_
  openrouter_slug (got 'copilot')
- Root cause: ai-gateway, copilot, and kilocode are multi-vendor
  aggregators that list other providers' models (OpenRouter-style slugs).
  They were being matched in Step 1 before OpenRouter. Added all three
  to _AGGREGATORS set so they're skipped like nous/openrouter.

Group E (1 test): model_flow_custom StopIteration
- test_model_flow_custom_saves_verified_v1_base_url
- Root cause: 'Display name' prompt was added after the test was written.
  The input iterator had 5 answers but the flow now asks 6 questions.
  Added 6th empty string answer.

Group F (1 test): Telegram proxy env assertion
- test_uses_proxy_env_for_primary_and_fallback_transports
- Root cause: _resolve_proxy_url() now checks TELEGRAM_PROXY first
  (via resolve_proxy_url('TELEGRAM_PROXY')). Test didn't clear this
  env var, allowing potential leakage from other tests in xdist workers.
  Added TELEGRAM_PROXY to the cleanup list.

tests/cli/test_cli_provider_resolution.py

@@ -578,7 +578,7 @@ def test_model_flow_custom_saves_verified_v1_base_url(monkeypatch, capsys):
     # After the probe detects a single model ("llm"), the flow asks
     # "Use this model? [Y/n]:" — confirm with Enter, then context length,
     # then display name.
-    answers = iter(["http://localhost:8000", "local-key", "", "", ""])
+    answers = iter(["http://localhost:8000", "local-key", "", "", "", ""])
     monkeypatch.setattr("builtins.input", lambda _prompt="": next(answers))
     monkeypatch.setattr("getpass.getpass", lambda _prompt="": next(answers))