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/tool_execution.py

@@ -21,6 +21,7 @@ from typing import Any, Awaitable, Callable, Dict, Optional, Tuple
 from src.tool_security import is_public_blocked_tool, owner_is_admin_or_single_user
 from src.tool_policy import ToolPolicy
 from src.constants import MAX_OUTPUT_CHARS, MAX_READ_CHARS, MAX_DIFF_LINES, DATA_DIR
+from src.tool_utils import _truncate, get_mcp_manager
 
 # Persistent working directory for agent subprocesses.
 # Resolves to <repo_root>/data, which is the bind-mounted volume in Docker
@@ -326,12 +327,6 @@ PROGRESS_INTERVAL_S = 2.0
 # snippet without dragging the whole output along.
 PROGRESS_TAIL_LINES = 12
 
-
-def get_mcp_manager():
-    from src import agent_tools
-    return agent_tools.get_mcp_manager()
-
-
 # Directories ignored by the code-nav tools' Python fallbacks so results aren't
 # polluted by VCS internals / dependency trees / build caches. ripgrep already
 # honours .gitignore; this is the parity floor for the no-rg path (and the
@@ -364,12 +359,6 @@ def _resolve_search_root(raw_path: str, workspace: Optional[str] = None) -> str:
         return roots[0] if roots else os.path.realpath(".")
     return _resolve_tool_path(raw)
 
-
-def _truncate(text: str, limit: int = MAX_OUTPUT_CHARS) -> str:
-    if len(text) > limit:
-        return text[:limit] + f"\n... (truncated, {len(text)} chars total)"
-    return text
-
 logger = logging.getLogger(__name__)