fix: default session mode experiment not applying on first session

[andysharman]

Fix: default session mode not applying on first session

Follows up on #306532.

The chat.newSession.defaultMode experiment wasn't applying because TAS and custom modes (like Plan) load asynchronously — after
the first session was already created.

Changes:

• Remove static config registration that competed with the TAS-driven default
• Add listeners to re-apply the default mode when the TAS config and custom modes become available

Both listeners are scoped to the session lifecycle and only fire while the session is empty.

[Copilot]

Pull request overview

Fixes the “default new session mode” A/B experiment not applying for the first created chat session by ensuring the default mode is re-applied once asynchronous dependencies (TAS-registered setting and custom modes) become available.

Changes:

• Stop statically registering chat.newSession.defaultMode so it no longer competes with the TAS-driven default.
• When a session is empty, listen for config/mode availability changes and re-run the empty-session initialization logic to apply the intended default mode.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/browser/widget/input/chatInputPart.ts	Adds session-scoped listeners to re-apply empty-session defaults when the setting/modes become available.
src/vs/workbench/contrib/chat/browser/chat.contribution.ts	Removes the static registration of chat.newSession.defaultMode to avoid competing defaults with TAS-driven registration.

Copilot's findings

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

[Copilot]

These listeners call _setEmptyModelState() whenever the default-mode setting becomes available or chat modes change, as long as _chatSessionIsEmpty is true. That can override a mode the user manually selected before sending their first message (the session is still “empty”, but the user intent is already captured in the input model).

Consider gating this re-apply so it only runs if the user hasn’t changed mode/input state yet (e.g., only when the input model has no state yet, or track a "defaultApplied"/"userInteracted" flag).

Suggested change

	if (chatSessionIsEmpty) {
	this._setEmptyModelState();
	// The default mode setting may be registered asynchronously by TAS,
	// and custom modes (like Plan) load asynchronously from prompt files.
	// Re-apply when either becomes available.
	this._modelSyncDisposables.add(this.configurationService.onDidChangeConfiguration(e => {
	if (this._chatSessionIsEmpty && e.affectsConfiguration(ChatConfiguration.DefaultNewSessionMode)) {
	this._setEmptyModelState();
	}
	}));
	this._modelSyncDisposables.add(this.chatModeService.onDidChangeChatModes(() => {
	if (this._chatSessionIsEmpty) {
	this._setEmptyModelState();
	}
	const canApplyEmptyModelState = () => {
	return this._chatSessionIsEmpty && this._inputModel === model && !model.state.get();
	};
	const applyEmptyModelStateIfNeeded = () => {
	if (canApplyEmptyModelState()) {
	this._setEmptyModelState();
	}
	};
	if (chatSessionIsEmpty) {
	applyEmptyModelStateIfNeeded();
	// The default mode setting may be registered asynchronously by TAS,
	// and custom modes (like Plan) load asynchronously from prompt files.
	// Re-apply when either becomes available, but only while the input
	// model still has no explicit state of its own.
	this._modelSyncDisposables.add(this.configurationService.onDidChangeConfiguration(e => {
	if (e.affectsConfiguration(ChatConfiguration.DefaultNewSessionMode)) {
	applyEmptyModelStateIfNeeded();
	}
	}));
	this._modelSyncDisposables.add(this.chatModeService.onDidChangeChatModes(() => {
	applyEmptyModelStateIfNeeded();