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_tools.py

@@ -14,7 +14,7 @@ Sub-modules:
 import logging
 from collections import namedtuple
 
-from src.constants import MAX_OUTPUT_CHARS, MAX_READ_CHARS
+from src.tool_utils import _truncate, get_mcp_manager, set_mcp_manager
 
 logger = logging.getLogger(__name__)
 
@@ -64,33 +64,6 @@ TOOL_TAGS = {"bash", "python", "web_search", "web_fetch", "read_file", "write_fi
 
 ToolBlock = namedtuple("ToolBlock", ["tool_type", "content"])
 
-# ---------------------------------------------------------------------------
-# MCP Manager (kept here — used by execution and agent_loop)
-# ---------------------------------------------------------------------------
-_mcp_manager = None
-
-def set_mcp_manager(manager):
-    """Set the global MCP manager instance."""
-    global _mcp_manager
-    _mcp_manager = manager
-
-def get_mcp_manager():
-    """Get the global MCP manager instance."""
-    return _mcp_manager
-
-# ---------------------------------------------------------------------------
-# Helpers (kept here — used by sub-modules)
-# ---------------------------------------------------------------------------
-def _truncate(text: str, limit: int = MAX_OUTPUT_CHARS) -> str:
-    # Callers treat the result as text, so always return a string: coerce a
-    # non-string (None -> "", otherwise str(...)) instead of returning it raw,
-    # which would just move the crash downstream.
-    if not isinstance(text, str):
-        text = "" if text is None else str(text)
-    if len(text) > limit:
-        return text[:limit] + f"\n... (truncated, {len(text)} chars total)"
-    return text
-
 # ---------------------------------------------------------------------------
 # Re-exports from sub-modules
 # ---------------------------------------------------------------------------