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/agent_loop.py

@@ -21,6 +21,7 @@ from src.settings import get_setting
 from src.prompt_security import untrusted_context_message
 from src.tool_security import blocked_tools_for_owner, plan_mode_disabled_tools
 from src.tool_policy import GUIDE_ONLY_DIRECTIVE, ToolPolicy
+from src.tool_utils import get_mcp_manager
 from src.agent_tools import (
     parse_tool_blocks,
     strip_tool_blocks,
@@ -29,7 +30,6 @@ from src.agent_tools import (
     set_active_document,
     set_active_model,
     function_call_to_tool_block,
-    get_mcp_manager,
     FUNCTION_TOOL_SCHEMAS,
     TOOL_TAGS,
     ToolBlock,