Misleading error message when conversation context exceeds 20MB limit

Problem

When a conversation's accumulated context exceeds the 20MB request size limit, the error shown is:

Request too large (max 20MB). Try with a smaller file.

This is confusing because:

• The user may not have attached any file at all
• The error implies the fix is to use a smaller file, which doesn't help
• The session becomes stuck with no clear path forward — even typing "reset yourself" triggers the same error

Expected behavior

The error message should clearly indicate that the conversation history is too large, not a file, and suggest starting a new chat. For example:

Conversation context is too large to continue. Please start a new chat.

Steps to reproduce

1. Have a long conversation with many tool calls / large tool outputs
2. Eventually the accumulated context hits the 20MB limit
3. Every subsequent message returns the misleading error

Suggested improvement

• Change the error message to distinguish between "file too large" and "conversation context too large"
• Optionally, proactively warn the user as they approach the limit