refactor: replace litellm with native openai + anthropic SDKs

- Remove litellm dependency entirely (supply chain risk mitigation)
- Add AnthropicProvider (native SDK) and OpenAICompatProvider (unified)
- Merge CustomProvider into OpenAICompatProvider, delete custom_provider.py
- Add ProviderSpec.backend field for declarative provider routing
- Remove _resolve_model, find_gateway, find_by_model (dead heuristics)
- Pass resolved spec directly into provider — zero internal lookups
- Stub out litellm-dependent model database (cli/models.py)
- Add anthropic>=0.45.0 to dependencies, remove litellm
- 593 tests passed, net -1034 lines

tests/agent/test_gemini_thought_signature.py

@@ -1,40 +1,6 @@
 from types import SimpleNamespace
 
 from nanobot.providers.base import ToolCallRequest
-from nanobot.providers.litellm_provider import LiteLLMProvider
-
-
-def test_litellm_parse_response_preserves_tool_call_provider_fields() -> None:
-    provider = LiteLLMProvider(default_model="gemini/gemini-3-flash")
-
-    response = SimpleNamespace(
-        choices=[
-            SimpleNamespace(
-                finish_reason="tool_calls",
-                message=SimpleNamespace(
-                    content=None,
-                    tool_calls=[
-                        SimpleNamespace(
-                            id="call_123",
-                            function=SimpleNamespace(
-                                name="read_file",
-                                arguments='{"path":"todo.md"}',
-                                provider_specific_fields={"inner": "value"},
-                            ),
-                            provider_specific_fields={"thought_signature": "signed-token"},
-                        )
-                    ],
-                ),
-            )
-        ],
-        usage=None,
-    )
-
-    parsed = provider._parse_response(response)
-
-    assert len(parsed.tool_calls) == 1
-    assert parsed.tool_calls[0].provider_specific_fields == {"thought_signature": "signed-token"}
-    assert parsed.tool_calls[0].function_provider_specific_fields == {"inner": "value"}
 
 
 def test_tool_call_request_serializes_provider_fields() -> None:

tests/agent/test_memory_consolidation_types.py

@@ -380,7 +380,7 @@ class TestMemoryConsolidationTypeHandling:
         """Forced tool_choice rejected by provider -> retry with auto and succeed."""
         store = MemoryStore(tmp_path)
         error_resp = LLMResponse(
-            content="Error calling LLM: litellm.BadRequestError: "
+            content="Error calling LLM: BadRequestError: "
             "The tool_choice parameter does not support being set to required or object",
             finish_reason="error",
             tool_calls=[],