Forward response_id through StreamAccumulator to fix previous_response_id chaining

[rustemmukhamadiev]

Note

Saw the same fix already sitting in #12, but that PR's been open since
June and looks blocked on a bigger native-RubyLLM migration. Opening this
as a small standalone fix so it can land sooner — would love to see it
merged independently rather than wait on #12.

Bug

When using streaming (chat.ask(message, &chunk_handler)), the response_id from
the completed response never survives into the persisted message, so
previous_response_id chaining never engages on the next turn.

Root cause

Streaming.build_chunk correctly attaches response_id to the
response.completed chunk, but RubyLLM::StreamAccumulator#to_message
(core gem) builds the final Message without it, and
StreamAccumulatorExtension only forwarded built_in_tool_events
through the accumulator, not response_id.

Non-streaming (chat.complete/chat.ask without a block) is unaffected.

Fix

Track response_id the same way built_in_tool_events already is:
capture it in add, apply it in to_message.

Verified locally: after the patch, a second streamed turn correctly
sends previous_response_id and only the new message in input,
instead of resending the whole history.