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>

mcp_servers/_common.py

@@ -1,22 +0,0 @@
-"""
-_common.py
-
-Shared constants and helpers for built-in MCP servers.
-"""
-
-MAX_OUTPUT_CHARS = 10_000
-MAX_READ_CHARS = 20_000
-SHELL_TIMEOUT = 60
-PYTHON_TIMEOUT = 30
-SEARCH_TIMEOUT = 30
-
-
-def truncate(text: str, limit: int = MAX_OUTPUT_CHARS) -> str:
-    """Truncate text to *limit* characters with a suffix note."""
-    if not isinstance(text, str):
-        # Tool output is occasionally None or a non-string; len(None) would
-        # raise. Coerce so this shared helper never crashes a tool response.
-        text = "" if text is None else str(text)
-    if len(text) > limit:
-        return text[:limit] + f"\n... (truncated, {len(text)} chars total)"
-    return text