fix(agents): make librarian failures structured and user-safe

- run_librarian / run_librarian_stream raise AgentError instead of
  returning/yielding error text as normal output; detail stays in logs
- delegate_to_* wrappers now put a curated butler-toned sentence in
  DelegationResult.output on failure and never expose str(e), so
  streaming's error branch is reachable and honest
- _execute_single_delegation propagates success; direct delegation only
  records delegate_to_* as called when the expert actually succeeded
- librarian tools return user-safe messages instead of
  'Error searching: {e}' strings that leaked internal URLs into
  synthesis; coordination stream errors are curated as well
- ruff cleanups (TYPE_CHECKING forward refs, B904, unused locals) in
  the touched files to keep them lint-clean

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/agents/librarian/test_tools.py

@@ -5,21 +5,20 @@ Tests the tool functions that wrap the Library-Desk API,
 including the new web search and content extraction tools.
 """
 
-import pytest
-from unittest.mock import AsyncMock, MagicMock, patch
+from unittest.mock import AsyncMock, patch
 
+import pytest
+
+from src.agents.librarian.client import (
+    BatchExtractionResponse,
+    ContentExtractionResult,
+    WebSearchResponse,
+    WebSearchResult,
+)
 from src.agents.librarian.tools import (
-    search_web,
     read_url,
     read_urls_batch,
-    hybrid_search,
-    search_wiki,
-)
-from src.agents.librarian.client import (
-    WebSearchResult,
-    WebSearchResponse,
-    ContentExtractionResult,
-    BatchExtractionResponse,
+    search_web,
 )
 
 
@@ -104,8 +103,10 @@ class TestSearchWeb:
 
     @pytest.mark.asyncio
     async def test_search_web_error_handling(self, mock_client):
-        """Test web search error handling."""
-        mock_client.search_web.side_effect = Exception("Connection failed")
+        """Test web search errors return a user-safe message without internals."""
+        mock_client.search_web.side_effect = Exception(
+            "Connection failed to http://internal-host:8089"
+        )
 
         with patch(
             "src.agents.librarian.tools.LibraryDeskClient"
@@ -115,8 +116,10 @@ class TestSearchWeb:
 
             result = await search_web("test query")
 
-            assert "Error" in result
-            assert "Connection failed" in result
+            assert "unable to search the web" in result
+            # Exception detail (internal URLs etc.) must not leak
+            assert "Connection failed" not in result
+            assert "internal-host" not in result
 
     @pytest.mark.asyncio
     async def test_search_web_with_news_type(self, mock_client):
@@ -227,7 +230,7 @@ class TestReadUrl:
             mock_client_class.return_value.__aenter__.return_value = mock_client
             mock_client_class.return_value.__aexit__.return_value = None
 
-            result = await read_url("https://example.com/long", max_length=2000)
+            await read_url("https://example.com/long", max_length=2000)
 
             mock_client.extract_content.assert_called_with(
                 url="https://example.com/long",