refactor(provider): use LiteLLM as unified LLM requester backend

- Replace 23+ individual requester implementations with unified litellmchat.py
  - Add litellm_provider field to 27 YAML manifests for provider routing
  - Delete redundant requester subclasses
  - Add unit tests for LiteLLMRequester (29 tests)
  - Fix num_retries parameter name (was max_retries)
  - Fix exception handling order for subclass exceptions

  LiteLLM provides unified API for 100+ providers, eliminating need for
  provider-specific requesters.

src/langbot/pkg/provider/modelmgr/requesters/chatcmpl.yaml

@@ -22,12 +22,10 @@ spec:
     type: integer
     required: true
     default: 120
+  # LiteLLM provider prefix - when set, uses unified LiteLLMRequester
+  litellm_provider: openai
   support_type:
   - llm
   - text-embedding
   - rerank
   provider_category: manufacturer
-execution:
-  python:
-    path: ./chatcmpl.py
-    attr: OpenAIChatCompletions