mcp(refactor[server_tools]): centralize socket_name exemption

The contract "every registered tool accepts socket_name except
list_servers" was encoded in two places: the prose of
``_BASE_INSTRUCTIONS`` (server.py) and a hardcoded set in
``test_registered_tools_accept_socket_name`` (tests/test_server.py).
A second exempt tool would have required two synchronized edits.

Lift the exemption to a module-level constant
``SOCKET_NAME_EXEMPT: frozenset[str]`` next to ``list_servers`` in
``server_tools.py``. The test imports it; the docstring near the
constant tells future contributors to update the
``_BASE_INSTRUCTIONS`` prose alongside any addition.

No behaviour change.

Author: tony

src/libtmux_mcp/tools/server_tools.py

@@ -259,6 +259,16 @@ def _probe_server_by_path(socket_path: pathlib.Path) -> ServerInfo | None:
     )
 
 
+#: Tools that intentionally do NOT accept ``socket_name`` because they
+#: discover or enumerate sockets themselves rather than connecting to a
+#: known one. Read by ``test_registered_tools_accept_socket_name`` to
+#: enforce the agent-facing contract advertised in
+#: :data:`libtmux_mcp.server._BASE_INSTRUCTIONS`. When you add a new
+#: discovery-style tool, append it here AND update the prose in
+#: ``_BASE_INSTRUCTIONS`` so the two stay in lockstep.
+SOCKET_NAME_EXEMPT: frozenset[str] = frozenset({"list_servers"})
+
+
 @handle_tool_errors
 def list_servers(
     extra_socket_paths: list[str] | None = None,

tests/test_server.py

@@ -207,16 +207,15 @@ def test_registered_tools_accept_socket_name() -> None:
     from fastmcp.tools.function_tool import FunctionTool
 
     from libtmux_mcp.tools import register_tools
-
-    socket_name_exempt = {"list_servers"}
+    from libtmux_mcp.tools.server_tools import SOCKET_NAME_EXEMPT
 
     mcp = FastMCP(name="socket-name-contract")
     register_tools(mcp)
 
     tools = asyncio.run(mcp.list_tools())
     assert tools, "register_tools should have registered at least one tool"
     for tool in tools:
-        if tool.name in socket_name_exempt:
+        if tool.name in SOCKET_NAME_EXEMPT:
             continue
         assert isinstance(tool, FunctionTool), (
             f"Tool {tool.name!r} is not a FunctionTool; the registry "
@@ -226,7 +225,8 @@ def test_registered_tools_accept_socket_name() -> None:
         sig = inspect.signature(tool.fn)
         assert "socket_name" in sig.parameters, (
             f"Tool {tool.name!r} omits socket_name; either add it, "
-            f"add to socket_name_exempt, or update _BASE_INSTRUCTIONS"
+            f"add to server_tools.SOCKET_NAME_EXEMPT, or update "
+            f"_BASE_INSTRUCTIONS"
         )