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>

tests/test_tool_utils_import_clean.py

@@ -0,0 +1,22 @@
+"""Verify src.tool_utils has no project imports beyond src.constants.
+
+If someone adds an import from src.settings, src.database, or any other
+project module inside tool_utils.py, the circular import that this module
+exists to break will silently return a partially-initialized module.
+This test catches that statically.
+"""
+
+import ast
+import pathlib
+
+
+def test_tool_utils_has_no_project_imports():
+    src = pathlib.Path("src/tool_utils.py").read_text()
+    tree = ast.parse(src)
+    for node in ast.walk(tree):
+        if isinstance(node, (ast.Import, ast.ImportFrom)):
+            if isinstance(node, ast.ImportFrom) and node.module:
+                msg = f"Illegal project import in tool_utils.py: {node.module}"
+                assert node.module in ("src.constants",) or not node.module.startswith(
+                    "src."
+                ), msg