Prevent chat tool-call rows from overflowing the message column

[mgoldsborough]

Summary

• Assistant message wrapper was sizing to its widest child because the parent flex column uses items-start. Tool-row summaries use white-space: nowrap, so their intrinsic width blew out the wrapper and the existing row-summary ellipsis never engaged.
• Added w-full to the wrapper so it pins to the message column width — overflow-hidden on the wrapper plus text-overflow: ellipsis on .tool-accordion__row-summary now take effect.

Test plan

• Open a chat with a multi-tool-call block where at least one tool has a long first argument (e.g. a long SQL question)
• Confirm the row summary truncates with an ellipsis at the column edge instead of overflowing
• Check both full-screen chat and compact/sidebar modes
• Confirm prose and single-tool headers still render as before