telemetry(chat): agentic chat metrics

[chungjac]

Problem

• currently no metrics for agentic chat

Solution

Emit telemetry events when:

• message shown to user (message sent from user already exists)
• diff was generated
• user rejected diff
• command was generated
• user accepted command
• user pressed stop button
• upstreamed metric to aws-toolkit-common: telemetry(amazonq): intial metric for agentic chat aws-toolkit-common#1010

TODO:

• additional telemetry events for: user interrupts Q while generating, thumbs up/down
• see when user pressed stop (ex: chat, command execution, etc)

---

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[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.