fix: align empty auth token error message with test expectations

Update error message for empty authentication tokens to match test expectations.
This ensures consistency between the validation logic and test suite.

Change-Id: I473d1379fe0362d41384f5f88128b759ad8ae685
Signed-off-by: Thomas Kosiewski <tk@coder.com>

Author: ThomasK33

lua/claudecode/server/handshake.lua

@@ -48,7 +48,7 @@ function M.validate_upgrade_request(request, expected_auth_token)
 
     -- Check for empty auth header
     if auth_header == "" then
-      return false, "Empty authentication token provided"
+      return false, "Authentication token too short (min 10 characters)"
     end
 
     -- Check for suspicious auth header lengths