fix(gateway): normalize step_callback prev_tools for backward compat

The PR changed prev_tools from list[str] to list[dict] with name/result
keys.  The gateway's _step_callback_sync passed this directly to hooks
as 'tool_names', breaking user-authored hooks that call
', '.join(tool_names).

Now:
- 'tool_names' always contains strings (backward-compatible)
- 'tools' carries the enriched dicts for hooks that want results

Also adds summary logging to register_mcp_servers() and comprehensive
tests for all three PR changes:
- sanitize_mcp_name_component edge cases
- register_mcp_servers public API
- _register_session_mcp_servers ACP integration
- step_callback result forwarding
- gateway normalization backward compat

tests/gateway/test_step_callback_compat.py

@@ -0,0 +1,133 @@
+"""Tests for step_callback backward compatibility.
+
+Verifies that the gateway's step_callback normalization keeps
+``tool_names`` as a list of strings for backward-compatible hooks,
+while also providing the enriched ``tools`` list with results.
+"""
+
+import asyncio
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+
+class TestStepCallbackNormalization:
+    """The gateway's _step_callback_sync normalizes prev_tools from run_agent."""
+
+    def _extract_step_callback(self):
+        """Build a minimal _step_callback_sync using the same logic as gateway/run.py.
+
+        We replicate the closure so we can test normalisation in isolation
+        without spinning up the full gateway.
+        """
+        captured_events = []
+
+        class FakeHooks:
+            async def emit(self, event_type, data):
+                captured_events.append((event_type, data))
+
+        hooks_ref = FakeHooks()
+        loop = asyncio.new_event_loop()
+
+        def _step_callback_sync(iteration: int, prev_tools: list) -> None:
+            _names: list[str] = []
+            for _t in (prev_tools or []):
+                if isinstance(_t, dict):
+                    _names.append(_t.get("name") or "")
+                else:
+                    _names.append(str(_t))
+            asyncio.run_coroutine_threadsafe(
+                hooks_ref.emit("agent:step", {
+                    "iteration": iteration,
+                    "tool_names": _names,
+                    "tools": prev_tools,
+                }),
+                loop,
+            )
+
+        return _step_callback_sync, captured_events, loop
+
+    def test_dict_prev_tools_produce_string_tool_names(self):
+        """When prev_tools is list[dict], tool_names should be list[str]."""
+        cb, events, loop = self._extract_step_callback()
+
+        # Simulate the enriched format from run_agent.py
+        prev_tools = [
+            {"name": "terminal", "result": '{"output": "hello"}'},
+            {"name": "read_file", "result": '{"content": "..."}'},
+        ]
+
+        try:
+            loop.run_until_complete(asyncio.sleep(0))  # prime the loop
+            import threading
+            t = threading.Thread(target=cb, args=(1, prev_tools))
+            t.start()
+            t.join(timeout=2)
+            loop.run_until_complete(asyncio.sleep(0.1))
+        finally:
+            loop.close()
+
+        assert len(events) == 1
+        _, data = events[0]
+        # tool_names must be strings for backward compat
+        assert data["tool_names"] == ["terminal", "read_file"]
+        assert all(isinstance(n, str) for n in data["tool_names"])
+        # tools should be the enriched dicts
+        assert data["tools"] == prev_tools
+
+    def test_string_prev_tools_still_work(self):
+        """When prev_tools is list[str] (legacy), tool_names should pass through."""
+        cb, events, loop = self._extract_step_callback()
+
+        prev_tools = ["terminal", "read_file"]
+
+        try:
+            loop.run_until_complete(asyncio.sleep(0))
+            import threading
+            t = threading.Thread(target=cb, args=(2, prev_tools))
+            t.start()
+            t.join(timeout=2)
+            loop.run_until_complete(asyncio.sleep(0.1))
+        finally:
+            loop.close()
+
+        assert len(events) == 1
+        _, data = events[0]
+        assert data["tool_names"] == ["terminal", "read_file"]
+
+    def test_empty_prev_tools(self):
+        """Empty or None prev_tools should produce empty tool_names."""
+        cb, events, loop = self._extract_step_callback()
+
+        try:
+            loop.run_until_complete(asyncio.sleep(0))
+            import threading
+            t = threading.Thread(target=cb, args=(1, []))
+            t.start()
+            t.join(timeout=2)
+            loop.run_until_complete(asyncio.sleep(0.1))
+        finally:
+            loop.close()
+
+        assert len(events) == 1
+        _, data = events[0]
+        assert data["tool_names"] == []
+
+    def test_joinable_for_hook_example(self):
+        """The documented hook example: ', '.join(tool_names) should work."""
+        # This is the exact pattern from the docs
+        prev_tools = [
+            {"name": "terminal", "result": "ok"},
+            {"name": "web_search", "result": None},
+        ]
+
+        _names = []
+        for _t in prev_tools:
+            if isinstance(_t, dict):
+                _names.append(_t.get("name") or "")
+            else:
+                _names.append(str(_t))
+
+        # This must not raise — documented hook pattern
+        result = ", ".join(_names)
+        assert result == "terminal, web_search"