fix(security): prevent shell injection in tilde-username path expansion

Validate that the username portion of ~username paths contains only
valid characters (alphanumeric, dot, hyphen, underscore) before passing
to shell echo for expansion. Previously, paths like '~; rm -rf /'
would be passed unquoted to self._exec(f'echo {path}'), allowing
arbitrary command execution.

The approach validates the username rather than using shlex.quote(),
which would prevent tilde expansion from working at all since
echo '~user' outputs the literal string instead of expanding it.

Added tests for injection blocking and valid ~username/path expansion.

Credit to @alireza78a for reporting (PR #442, issue #442).

tools/file_operations.py

@@ -400,10 +400,16 @@ class ShellFileOperations(FileOperations):
                     return home
                 elif path.startswith('~/'):
                     return home + path[1:]  # Replace ~ with home
-                # ~username format - let shell expand it
-                expand_result = self._exec(f"echo {path}")
-                if expand_result.exit_code == 0:
-                    return expand_result.stdout.strip()
+                # ~username format - extract and validate username before
+                # letting shell expand it (prevent shell injection via
+                # paths like "~; rm -rf /").
+                rest = path[1:]  # strip leading ~
+                slash_idx = rest.find('/')
+                username = rest[:slash_idx] if slash_idx >= 0 else rest
+                if username and re.fullmatch(r'[a-zA-Z0-9._-]+', username):
+                    expand_result = self._exec(f"echo {path}")
+                    if expand_result.exit_code == 0 and expand_result.stdout.strip():
+                        return expand_result.stdout.strip()
         
         return path