feat(amazonq): improve explain functionality

[BlakeLazarine]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
  Prompt changes

Description

Customers complained about missing the old deterministic message which was stored in the recommendation.text field.

As the agent to include this message at the start of its explain response.

This also is reaching parity with the VS Code Plugin's explain and fix prompting
https://github.com/aws/aws-toolkit-vscode/blob/master/packages/amazonq/src/lsp/chat/commands.ts

Checklist

• My code follows the code style of this project
• I have added tests to cover my changes
• A short description of the change has been added to the CHANGELOG if the change is customer-facing in the IDE.
• I have added metrics for my changes (if required)

License

I confirm that my contribution is made under the terms of the Apache 2.0 license.