Add model managed setting and make it default for new conversations

[vijayupadya]

Fixes https://github.com/microsoft/vscode-internalbacklog/issues/7934

Adds support for configuring a default model that applies to every new conversation, enforable by enterprise policy.

What's new

• New model property on the chat.defaultModel setting. Accepts:

  • "auto" — let Copilot pick the model
  • a model family name (e.g. "opus", "gemini") — resolves to the latest available version in that family
  • a full model id

• A new ChatDefaultModel enterprise policy that maps the model managed setting onto this configuration.

Behavior

• New conversations start at the configured model across the local chat panel and the Agents window (including Copilot CLI sessions).
• Users can still switch models within a conversation; an explicit pick is never overridden by the configured default.
• Reopened conversations keep their own saved model.
• When the setting is unset, behavior is unchanged.
• Setting applied to both local agent and AH

[Copilot]

Pull request overview

This pull request adds support for configuring a default chat model for new conversations via chat.defaultConfiguration.model, including enterprise enforcement via a new managed-setting–backed policy. The change is applied across the workbench chat widget and the Agents window model picker, with shared resolution logic for auto, exact model IDs, and model-family-to-latest selection.

Changes:

• Add chat.defaultConfiguration.model (string) and implement resolution of configured values to a concrete model (auto / full ID / family→latest).
• Apply the configured default model to new/empty sessions in the workbench chat input and in the Agents window picker, while tracking explicit user picks to avoid clobbering.
• Introduce a new ChatDefaultModel enterprise policy backed by a new managed-settings key (model) and update exported policy data.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/test/browser/widget/input/chatModelSelectionLogic.test.ts	Adds unit tests for configured-model resolution behavior (empty/auto/id/family/precedence).
src/vs/workbench/contrib/chat/common/constants.ts	Extends IChatDefaultConfiguration with model?: string and updates documentation.
src/vs/workbench/contrib/chat/browser/widget/input/chatModelSelectionLogic.ts	Implements resolveConfiguredModel (and version comparison) to map configured values to available models.
src/vs/workbench/contrib/chat/browser/widget/input/chatInputPart.ts	Applies configured defaults to new/empty sessions, waits for late model registration, and tracks explicit user selections.
src/vs/workbench/contrib/chat/browser/chat.shared.contribution.ts	Registers the new configuration property and attaches a managed-settings–driven enterprise policy.
src/vs/workbench/contrib/chat/browser/actions/chatActions.ts	Marks model selection triggered by action options as user-initiated.
src/vs/sessions/contrib/chat/browser/modelPicker.ts	Applies the configured default model when initializing model selection for new sessions in the Agents window.
src/vs/platform/policy/common/copilotManagedSettings.ts	Adds COPILOT_MODEL_KEY managed-settings key constant.
build/lib/policies/policyData.jsonc	Updates the generated policy catalog to include ChatDefaultModel.

Copilot's findings

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/browser/widget/input/chatInputPart.ts:1553

• Spelling: the debug log message uses modelDetials which makes log searching harder (and is inconsistent with the variable name).

		const selectedModelStorageKey = this.getSelectedModelStorageKey();
		const selectedModelIsDefaultStorageKey = this.getSelectedModelIsDefaultStorageKey();
		logChangesToStateModel(this._inputModel, `setCurrentLanguageModel to ${model.identifier} in ${this._currentSessionKey}, storageKey=${selectedModelStorageKey}, isDefaultKey=${selectedModelIsDefaultStorageKey}, currentSessionType=${this._currentSessionType}, getCurrentSessionType=${this.getCurrentSessionType()}, boundInputModelSession=${this._inputModelSessionResource?.toString()}, modelDetials = ${modelDetails}`, undefined, undefined, this.logService);

• Files reviewed: 9/9 changed files
• Comments generated: 6

[Copilot]

Copilot's findings

• Files reviewed: 11/11 changed files
• Comments generated: 2

[vijayupadya]

cc @joshspicer