Refactor models and logging for nexus-claude-api

- Update default models in config to use `claude-opus-4.6`.
- Introduce logging configuration to write logs to a file.
- Add correlation ID to error responses for better traceability.
- Implement diagnostics for summarizing message requests.
- Normalize legacy system messages in the API.
- Enhance tests to cover new logging and error handling features.
- Update README and documentation to reflect changes in model defaults and logging behavior.

tests/test_diagnostics.py

@@ -0,0 +1,95 @@
+from __future__ import annotations
+
+import base64
+
+from nexus_claude_api.diagnostics import summarize_messages_request
+from nexus_claude_api.models import AnthropicMessagesRequest
+
+
+def test_text_request_summary_omits_prompt_text() -> None:
+    payload = AnthropicMessagesRequest.model_validate(
+        {
+            "model": "claude-opus-4.6",
+            "messages": [{"role": "user", "content": "secret prompt"}],
+            "max_tokens": 32,
+        }
+    )
+
+    summary = summarize_messages_request(payload, correlation_id="corr-1")
+
+    assert summary["correlation_id"] == "corr-1"
+    assert summary["model"] == "claude-opus-4.6"
+    assert summary["backend_model"] == "claude-opus-4.6"
+    assert summary["stream"] is False
+    assert summary["message_count"] == 1
+    assert summary["content_block_types"] == {"text": 1}
+    assert "secret prompt" not in repr(summary)
+
+
+def test_image_request_summary_omits_base64_and_records_size() -> None:
+    image_data = base64.b64encode(b"image-bytes").decode("ascii")
+    payload = AnthropicMessagesRequest.model_validate(
+        {
+            "model": "claude-opus-4.6",
+            "messages": [
+                {
+                    "role": "user",
+                    "content": [
+                        {"type": "text", "text": "describe"},
+                        {
+                            "type": "image",
+                            "source": {
+                                "type": "base64",
+                                "media_type": "image/png",
+                                "data": image_data,
+                            },
+                        },
+                    ],
+                }
+            ],
+            "max_tokens": 32,
+        }
+    )
+
+    summary = summarize_messages_request(payload, correlation_id="corr-2")
+
+    assert summary["content_block_types"] == {"image": 1, "text": 1}
+    assert summary["images"] == [{"media_type": "image/png", "byte_size": 11}]
+    assert image_data not in repr(summary)
+    assert "image-bytes" not in repr(summary)
+
+
+def test_tool_request_summary_omits_tool_inputs() -> None:
+    payload = AnthropicMessagesRequest.model_validate(
+        {
+            "model": "claude-opus-4.6",
+            "messages": [
+                {
+                    "role": "assistant",
+                    "content": [
+                        {
+                            "type": "tool_use",
+                            "id": "toolu_1",
+                            "name": "lookup_secret",
+                            "input": {"api_key": "secret-tool-input"},
+                        }
+                    ],
+                }
+            ],
+            "tools": [
+                {
+                    "name": "lookup_secret",
+                    "input_schema": {"type": "object"},
+                }
+            ],
+            "tool_choice": {"type": "tool", "name": "lookup_secret"},
+            "max_tokens": 32,
+        }
+    )
+
+    summary = summarize_messages_request(payload, correlation_id="corr-3")
+
+    assert summary["tools"] == {"count": 1, "names": ["lookup_secret"]}
+    assert summary["tool_choice_type"] == "tool"
+    assert "secret-tool-input" not in repr(summary)
+    assert "api_key" not in repr(summary)