fix: reset chat mode when switching to session with customAgentTarget

[DonJayamanne]

Fixes #307118

When switching from Local to CLI, the Ask/Plan mode incorrectly persisted because the ModePickerActionItem captured customAgentTarget once at construction time and used it to statically select the action provider for the dropdown. Since the widget isn't recreated on session switch, the dropdown kept showing Local's full mode list (Ask/Edit/Agent) even in CLI. Additionally, chatInputPart validated the selected model on session switch but never validated the mode.

This PR makes the mode picker dynamically evaluate the customAgentTarget each time the dropdown is opened, and adds a checkModeInSessionPool() method (analogous to the existing checkModelInSessionPool()) that resets invalid modes to Agent when switching to a session type with a customAgentTarget.

How to test:

1. Open Copilot Chat, switch to "Local" provider
2. Select Ask or Plan mode
3. Switch to CLI provider
4. Verify the mode resets to Agent and the dropdown only shows valid modes

[Copilot]

Pull request overview

Fixes an issue where chat mode (Ask/Plan/custom agents) could incorrectly persist when switching to a session type that uses customAgentTarget (e.g. Local → CLI), by making the mode picker target-aware at open time and adding mode validation on session/provider changes.

Changes:

• Make ModePickerActionItem choose its dropdown action provider dynamically based on the current customAgentTarget.
• Add checkModeInSessionPool() to reset invalid modes to Agent when entering a session type with a customAgentTarget.
• Invoke mode validation on session type/provider changes alongside existing model validation.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/browser/widget/input/modePickerActionItem.ts	Dynamically selects the action provider at dropdown open time so mode lists reflect the current customAgentTarget.
src/vs/workbench/contrib/chat/browser/widget/input/chatInputPart.ts	Adds and wires checkModeInSessionPool() to reset invalid modes when switching session/provider context.

Copilot's findings

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/browser/widget/input/chatInputPart.ts:1963

• checkModeInSessionPool() runs before preselectModelFromSessionHistory(), but preselectModelFromSessionHistory() can change the mode (via modeInfo.modeInstructions.uri). If the last request in the new session references a mode that is invalid for the session’s customAgentTarget, the mode will be re-set after this validation and the UI can still end up in an invalid mode. Consider running checkModeInSessionPool() after preselectModelFromSessionHistory() (or re-validating immediately after setting the mode there).

			const newSessionType = this.getCurrentSessionType();
			if (e.currentSessionResource && newSessionType !== this._currentSessionType) {
				this._currentSessionType = newSessionType;
				this.initSelectedModel();
				this.checkModelInSessionPool();
				this.checkModeInSessionPool();
			}

			// For contributed sessions with history, pre-select the model
			// from the last request so the user resumes with the same model.
			this.preselectModelFromSessionHistory();

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