chat: combine multiple pending steering messages into single request

[connor4312]

When Copilot is running a tool, sending multiple steering messages only sends
the first one. The chatbot waits indefinitely for the remaining messages.

This fix combines all consecutive pending steering messages into a single
request by:

• Adding dequeueAllSteeringRequests() to ChatModel to dequeue all
  consecutive steering messages at the front of the queue
• Refactoring processNextPendingRequest() to handle both steering and queued
  requests through a unified flow that:
  • Combines multiple steering message texts with \n\n separator
  • Merges attachments from all steering messages
  • Re-parses the combined text
  • Sends as a single request to the agent

Fixes #298324

(Commit message generated by Copilot)

[Copilot]

Pull request overview

Fixes an issue in the chat request queue where multiple pending steering messages (queued while a tool/run is in progress) would not all be sent, causing later steering messages to remain stuck indefinitely. The PR introduces logic to dequeue and coalesce consecutive steering messages into a single outgoing request.

Changes:

• Added ChatModel.dequeueAllSteeringRequests() to dequeue consecutive steering requests at the front of the pending queue.
• Refactored ChatService.processNextPendingRequest() to combine multiple steering messages (text + attachments) and send them as one request.
• Added a unit test covering the “multiple steering messages get combined” behavior.

Reviewed changes

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

File	Description
src/vs/workbench/contrib/chat/test/common/chatService/chatService.test.ts	Adds a test ensuring multiple queued steering messages result in a single follow-up agent invocation.
src/vs/workbench/contrib/chat/common/model/chatModel.ts	Adds a helper to dequeue all consecutive steering requests from the front of the queue.
src/vs/workbench/contrib/chat/common/chatService/chatServiceImpl.ts	Updates pending-request processing to coalesce consecutive steering requests and resolve all queued deferreds.

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