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/run_agent/test_context_token_tracking.py

@@ -59,7 +59,7 @@ def _make_agent(monkeypatch, api_mode, provider, response_fn):
             self._disable_streaming = True
             return super().run_conversation(msg, conversation_history=conversation_history, task_id=task_id)
 
-    return _A(model="test-model", api_key="test-key", provider=provider, api_mode=api_mode)
+    return _A(model="test-model", api_key="test-key", base_url="http://localhost:1234/v1", provider=provider, api_mode=api_mode)
 
 
 def _anthropic_resp(input_tok, output_tok, cache_read=0, cache_creation=0):