Overview tool: raise ValueError for directory paths (if multiple LS are active)

The error is raised through `LanguageServerManager.get_language_server`, where it is done conservatively (only if multiple LS are available and we can't decide)

src/serena/ls_manager.py

@@ -1,4 +1,5 @@
 import logging
+import os.path
 import threading
 from collections.abc import Iterator
 
@@ -129,8 +130,11 @@ class LanguageServerManager:
         return ls
 
     def get_language_server(self, relative_path: str) -> SolidLanguageServer:
+        """:param relative_path: relative path to a file"""
         ls: SolidLanguageServer | None = None
         if len(self._language_servers) > 1:
+            if os.path.isdir(relative_path):
+                raise ValueError(f"Expected a file path, but got a directory: {relative_path}")
             for candidate in self._language_servers.values():
                 if not candidate.is_ignored_path(relative_path, ignore_unsupported_files=True):
                     ls = candidate

src/serena/symbol.py

@@ -697,6 +697,7 @@ class LanguageServerSymbolRetriever:
         return self._ls_manager.get_root_path()
 
     def get_language_server(self, relative_path: str) -> SolidLanguageServer:
+        """:param relative_path: relative path to a file"""
         return self._ls_manager.get_language_server(relative_path)
 
     def find(
@@ -837,7 +838,7 @@ class LanguageServerSymbolRetriever:
 
     def get_symbol_overview(self, relative_path: str) -> dict[str, list[LanguageServerSymbol]]:
         """
-        :param relative_path: the path of the file or directory for which to get the symbol overview
+        :param relative_path: the path of the file for which to get the symbol overview
         :return: a mapping from file paths to lists of symbols.
             For the case where a file is passed, the mapping will contain a single entry.
         """