refactor(tools): consolidate duplicated _truncate and get_mcp_manager into src/tool_utils (#3478)

* refactor(tools): consolidate duplicated _truncate and get_mcp_manager into src/tool_utils

Move all copies of _truncate(), get_mcp_manager(), and set_mcp_manager()
into a single leaf module (src/tool_utils.py) that imports only from
src.constants. This eliminates the lazy-import hack
('from src import agent_tools' inside function bodies) in tool_execution.py
and tool_implementations.py, and fixes a latent bug: the _truncate copy in
tool_execution.py was missing the isinstance guard and would crash on None.

Also deletes mcp_servers/_common.py — it was dead code with zero callers
anywhere in the codebase, containing its own copy of truncate() and
constants that already exist in src/constants.py.

* fix(tools): route remaining get_mcp_manager imports to src.tool_utils

The maintainer's feedback flagged src/task_scheduler.py:1857 and
routes/task_routes.py:977. A project-wide search found a third call site
in src/agent_loop.py that also imported get_mcp_manager from
src.agent_tools instead of src.tool_utils.

All three are now sourced from the canonical location in src.tool_utils.

---------

Co-authored-by: mcnoliveira <mcnoliveira@gmail.com>

src/task_scheduler.py

@@ -1098,7 +1098,7 @@ class TaskScheduler:
                                endpoint_url: str, model: str) -> str:
         """Gather raw data from all integrations, hand it to the LLM to write the check-in."""
         from src.tool_implementations import do_manage_notes
-        from src.agent_tools import get_mcp_manager
+        from src.tool_utils import get_mcp_manager
 
         tz_name = _resolve_task_timezone(db, task)
         try:
@@ -1854,7 +1854,7 @@ class TaskScheduler:
         have to special-case each tool's schema; the MCP tool ignores keys it
         doesn't recognise.
         """
-        from src.agent_tools import get_mcp_manager
+        from src.tool_utils import get_mcp_manager
         mcp = get_mcp_manager()
         if not mcp:
             logger.warning(f"Task {task.id}: MCP manager not available for delivery")