feat(amazonq): Add additonal fields for agentic chat in chat history

[tsmithsz]

Problem

Need to add additional fields to the chat history schema to support agentic chat

Solution

• Create new Message type to store all history fields
• Add messageToChatMessage converter
• Replace use of ChatItem type with Message type
• Use messageToChatMessage converter to create triggerPayload history

Testing

• Manually tested adding fields and verified json history contained the new fields

---

• Treat all work as PUBLIC. Private feature/x branches will not be squash-merged at release time.
• Your code changes must meet the guidelines in CONTRIBUTING.md.
• License: I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

• This pull request modifies code in src/* but no tests were added/updated.
  • Confirm whether tests should be added or ensure the PR description explains why tests are not required.

[github-actions]

• This pull request modifies code in src/* but no tests were added/updated.
  • Confirm whether tests should be added or ensure the PR description explains why tests are not required.
• This pull request implements a feat or fix, so it must include a changelog entry (unless the fix is for an unreleased feature). Review the changelog guidelines.
  • Note: beta or "experiment" features that have active users should announce fixes in the changelog.
  • If this is not a feature or fix, use an appropriate type from the title guidelines. For example, telemetry-only changes should use the telemetry type.

[avi-alpert]

convert back to chatitem for

aws-toolkit-vscode/packages/core/src/codewhispererChat/controllers/chat/tabBarController.ts

Line 90 in 91859e2

selectedTab.conversations.flatMap((conv: Conversation) => conv.messages),