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_mcp_common_truncate.py

@@ -1,27 +1,17 @@
-"""Regression: the shared MCP truncate() must tolerate non-string input."""
-import importlib.machinery
-import importlib.util
-from pathlib import Path
+"""Canonical _truncate must tolerate non-string input (regression).
 
-_PATH = Path(__file__).resolve().parents[1] / "mcp_servers" / "_common.py"
-
-
-def _load():
-    loader = importlib.machinery.SourceFileLoader("odysseus_mcp_common", str(_PATH))
-    spec = importlib.util.spec_from_loader(loader.name, loader)
-    module = importlib.util.module_from_spec(spec)
-    loader.exec_module(module)
-    return module
+Originally this tested mcp_servers/_common.py's copy, which was deleted
+since it had zero callers. Now it tests the canonical version.
+"""
 
+from src.tool_utils import _truncate
 
 def test_truncate_handles_none_and_nonstring():
-    c = _load()
-    assert c.truncate(None) == ""
-    assert c.truncate(12345) == "12345"
+    assert _truncate(None) == ""       # pyright: ignore[reportArgumentType]
+    assert _truncate(12345) == "12345" # pyright: ignore[reportArgumentType]
 
 
 def test_truncate_string_behaviour_unchanged():
-    c = _load()
-    assert c.truncate("hello", limit=100) == "hello"
-    out = c.truncate("x" * 50, limit=10)
+    assert _truncate("hello", limit=100) == "hello"
+    out = _truncate("x" * 50, limit=10)
     assert out.startswith("x" * 10) and "truncated" in out