Major refactor: API, providers, messaging, and Anthropic protocol

Consolidates the incremental refactor work into a single change set: modular web tools (api/web_tools), native Anthropic request building and SSE block policy, OpenAI conversion and error handling, provider transports and rate limiting, messaging handler and tree queue, safe logging, smoke tests, and broad test coverage.

tests/api/test_validation_log.py

@@ -0,0 +1,33 @@
+"""Tests for validation log summaries (metadata only)."""
+
+from api.validation_log import summarize_request_validation_body
+
+
+def test_summarize_lists_block_metadata_without_echoing_string_content():
+    body = {
+        "messages": [
+            {
+                "role": "user",
+                "content": "secret user phrase",
+            }
+        ],
+        "tools": [{"name": "web_search", "type": "web_search_20250305"}],
+    }
+    summary, tool_names = summarize_request_validation_body(body)
+    assert summary == [
+        {
+            "role": "user",
+            "content_kind": "str",
+            "content_length": 18,
+        }
+    ]
+    assert tool_names == ["web_search"]
+    blob = repr(summary) + repr(tool_names)
+    assert "secret" not in blob
+
+
+def test_summarize_handles_non_dict_messages_and_missing_tools():
+    body = {"messages": ["not_a_dict"]}
+    summary, tool_names = summarize_request_validation_body(body)
+    assert summary == [{"message_kind": "str"}]
+    assert tool_names == []