Defer Copilot Steering Consumption Signals

[connor4312]

This moves Copilot steering consumption signaling into CopilotAgentSession so steering messages stay visible until the Copilot SDK response resumes with a top-level tool call or the turn completes.

Summary:

• Deduplicate in-flight and already-consumed steering messages before sending to the SDK.
• Flush deferred steering_consumed signals on the next top-level tool start, turn completion, or abort.
• Add focused tests for deferred flushing, duplicate sends, and abort behavior.

Fixes #317877

[Copilot]

Pull request overview

Moves Copilot steering “consumed” signaling into CopilotAgentSession so steering messages remain visible until the SDK turn progresses (next top-level tool start), the turn completes, or the turn is aborted—addressing the UX issue where steering disappeared too early.

Changes:

• Defers steering_consumed emission by queueing consumed IDs and flushing them on top-level tool start, turn completion, or abort.
• Deduplicates steering sends by tracking in-flight and deferred-consumed steering message IDs.
• Updates/adds unit tests to validate deferred flushing behavior, duplicate suppression, and abort flushing.

Show a summary per file

File	Description
src/vs/platform/agentHost/node/copilot/copilotAgentSession.ts	Adds in-flight/consumed tracking and deferred flushing of steering_consumed on tool start/turn complete/abort.
src/vs/platform/agentHost/test/node/copilotAgentSession.test.ts	Updates sendSteering tests to assert deferred consumption timing and duplicate suppression behavior.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 1