Return {} for empty-string tool-call arguments in the interactions protocol - #911

Problem

Protocols::Interactions::Tools.parse_interaction_arguments runs JSON.parse on any String:

def parse_interaction_arguments(arguments)
  arguments.is_a?(String) ? JSON.parse(arguments) : arguments || {}
rescue JSON::ParserError => e
  raise ToolCallParseError.new(finish_reason: :tool_calls), cause: e
end

A function_call step whose arguments is an empty string ("") makes JSON.parse("") raise, so the whole run fails with ToolCallParseError for what is a valid zero-argument tool call.

The sibling parser already handles this — Protocols::ChatCompletions::Tools.parse_tool_call_arguments:

if arguments.nil? || arguments.empty?
  {}
else
  JSON.parse(arguments)
end

So the two tool-call parsers disagree on the same input.

Fix

Guard empty (and nil) arguments in parse_interaction_arguments the same way, before JSON.parse.

return {} if arguments.nil? || (arguments.is_a?(String) && arguments.empty?)

Tests

New spec/ruby_llm/protocols/interactions/tools_spec.rb: empty string → {}, missing key → {}, Hash passthrough, valid JSON object, malformed JSON → ToolCallParseError. The empty-string case raises on main.

bundle exec rspec spec/ruby_llm/protocols/interactions{_spec.rb,/tools_spec.rb} spec/ruby_llm/protocols/chat_completions/tools_spec.rb — green; rubocop clean on both files.