fix(chat): fix for history overload issue

[ashishrp-aws]

Problem

Overtime chat history added to request is exceeding 100 messages limit causing errors.

Solution

• Limiting history to 100 messages max per request.

---

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[tsmithsz]

This might cause invalid history.

We added a limit of 250 based on the API spec here:
https://github.com/aws/aws-toolkit-vscode/blob/feature/agentic-chat/packages/core/src/shared/db/chatDb/chatDb.ts#L28

I think the limit was increased a month ago