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_agent_failures.py

@@ -0,0 +1,60 @@
+"""
+Tests for structured failure behavior of The Librarian entry points.
+
+run_librarian and run_librarian_stream must raise AgentError on failure
+instead of returning/yielding error text as if it were research output,
+and the raised error must not leak exception detail (internal URLs etc.).
+"""
+
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+from src.agents.librarian.agent import run_librarian, run_librarian_stream
+from src.agents.protocol import AgentError
+
+
+@pytest.mark.unit
+class TestRunLibrarianFailures:
+    """run_librarian raises structured errors instead of returning text."""
+
+    @pytest.mark.asyncio
+    async def test_run_librarian_raises_agent_error(self):
+        """Failures raise AgentError rather than returning error prose."""
+        mock_agent = MagicMock()
+        mock_agent.run = AsyncMock(
+            side_effect=RuntimeError("Connection refused to http://internal:8089")
+        )
+
+        with patch(
+            "src.agents.librarian.agent.get_librarian_agent",
+            return_value=mock_agent,
+        ):
+            with pytest.raises(AgentError) as exc_info:
+                await run_librarian(task="Find Docker docs")
+
+        assert exc_info.value.agent_name == "librarian"
+        # Exception detail stays in logs only
+        assert "internal" not in str(exc_info.value)
+        assert "Connection refused" not in str(exc_info.value)
+
+    @pytest.mark.asyncio
+    async def test_run_librarian_stream_raises_agent_error(self):
+        """Streaming failures raise instead of yielding error text."""
+        mock_agent = MagicMock()
+        mock_agent.run_stream = MagicMock(
+            side_effect=RuntimeError("Connection refused to http://internal:8089")
+        )
+
+        with patch(
+            "src.agents.librarian.agent.get_librarian_agent",
+            return_value=mock_agent,
+        ):
+            collected: list[str] = []
+            with pytest.raises(AgentError) as exc_info:
+                async for delta in run_librarian_stream(task="Find Docker docs"):
+                    collected.append(delta)
+
+        assert exc_info.value.agent_name == "librarian"
+        assert collected == [], "no error text may be yielded as output"
+        assert "internal" not in str(exc_info.value)