feat(mcp): dynamic tool discovery via notifications/tools/list_changed (#3812)

When a connected MCP server sends a ToolListChangedNotification (per the
MCP spec), Hermes now automatically re-fetches the tool list, deregisters
removed tools, and registers new ones — without requiring a restart.

This enables MCP servers with dynamic toolsets (e.g. GitHub MCP with
GITHUB_DYNAMIC_TOOLSETS=1) to add/remove tools at runtime.

Changes:
- registry.py: add ToolRegistry.deregister() for nuke-and-repave refresh
- mcp_tool.py: extract _register_server_tools() from
  _discover_and_register_server() as a shared helper for both initial
  discovery and dynamic refresh
- mcp_tool.py: add _make_message_handler() and _refresh_tools() on
  MCPServerTask, wired into all 3 ClientSession sites (stdio, new HTTP,
  deprecated HTTP)
- Graceful degradation: silently falls back to static discovery when the
  MCP SDK lacks notification types or message_handler support
- 8 new tests covering registration, refresh, handler dispatch, and
  deregister

Salvaged from PR #1794 by shivvor2.

tools/registry.py

@@ -87,6 +87,23 @@ class ToolRegistry:
         if check_fn and toolset not in self._toolset_checks:
             self._toolset_checks[toolset] = check_fn
 
+    def deregister(self, name: str) -> None:
+        """Remove a tool from the registry.
+
+        Also cleans up the toolset check if no other tools remain in the
+        same toolset.  Used by MCP dynamic tool discovery to nuke-and-repave
+        when a server sends ``notifications/tools/list_changed``.
+        """
+        entry = self._tools.pop(name, None)
+        if entry is None:
+            return
+        # Drop the toolset check if this was the last tool in that toolset
+        if entry.toolset in self._toolset_checks and not any(
+            e.toolset == entry.toolset for e in self._tools.values()
+        ):
+            self._toolset_checks.pop(entry.toolset, None)
+        logger.debug("Deregistered tool: %s", name)
+
     # ------------------------------------------------------------------
     # Schema retrieval
     # ------------------------------------------------------------------