feat: add /summarize command to agent mode

[bharatvansh]

Adds a new /summarize command to Agent mode that triggers conversation history summarization using the existing SummarizedConversationHistory prompt infrastructure. The command returns ChatResult.metadata.summary so downstream components can store/use the generated summary.

What’s included

• Registers a new Agent command contribution:
  • Command: /summarize
  • Visibility gated by config.github.copilot.chat.summarizeAgentConversationHistory.enabled
  • Localized description via package.nls.json
• Implements /summarize handling in AgentIntent:
  • Excludes the current /summarize turn from the history being summarized
  • Normalizes existing summary metadata before summarizing
  • Produces a summary tied to the latest tool-call round (required for summary association)
  • Returns { metadata: { summary: { toolCallRoundId, text } } }
• Adds unit tests covering:
  • Successful summarization when enabled and there is tool-call history
  • Graceful behavior when the feature is disabled
  • Friendly message when there is nothing to summarize

User-facing behavior

• If summarization is disabled, the command responds with a message explaining how to enable it.
• If there is no prior history (or no tool calls to associate a summary with), the command responds with a friendly “nothing to summarize” message.
• On success, the command reports progress and returns summary metadata.

Testing

• npm run test:unit -- src/extension/intents/node/test/agentSummarizeCommand.spec.ts

[Copilot]

Pull request overview

This PR adds a new /summarize command to the agent mode chat participant, allowing users to manually trigger summarization of conversation history. The feature is controlled by a configuration setting and provides user-friendly feedback for various scenarios.

Changes:

• Added a /summarize command handler in AgentIntent that leverages the existing SummarizedConversationHistory component
• Implemented comprehensive unit tests covering enabled/disabled states, empty history, and successful summarization scenarios
• Registered the command in VS Code's contribution points with conditional visibility based on configuration

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/extension/intents/node/test/agentSummarizeCommand.spec.ts	New test file with 3 test cases covering different scenarios for the /summarize command
src/extension/intents/node/agentIntent.ts	Added handleRequest override and handleSummarizeCommand method to handle the new command
package.nls.json	Added localized description string for the summarize command
package.json	Registered the /summarize command with conditional visibility

[lramos15]

Similar to my exploration here #3137

[roblourens]

Thanks for the PR- this did not fully work when I started testing it but it was easy to fix up and I think this approach is fine.

[bhavyaus]

Thanks for this!