Fix context size handling for Agent Host

[pwang347]

Fixes #321707

Notes:

• Agent host context in context widget now correctly matches input + output tokens (we were setting output to always 0)
• Switching models will not also update the context widget, even if it wasn't actually used for the previous request

Dependent on AHP changes:

• Support json primitives for enums agent-host-protocol#257
• Add support for additional context properties  agent-host-protocol#258
• Widen model config to use JSON primitive agent-host-protocol#260

[Copilot]

Pull request overview

This PR fixes context size handling for Agent Host-backed chat models by representing the selected context window as a first-class ModelSelection.maxContextWindow value (token count) rather than smuggling it through the string-only config bag. It also introduces recommendedContextWindows on models so the chat UI can present a context-size picker consistently across agents, while letting each agent map the numeric selection to its backend representation (e.g. Copilot SDK contextTier).

Changes:

• Add recommendedContextWindows to model info and maxContextWindow to model selection in the agent-host protocol types, and plumb the new fields through side effects.
• Synthesize a tokens-group “Context Size” picker from recommendedContextWindows, and lift the selected numeric value into ModelSelection.maxContextWindow when creating sessions.
• Update Copilot agent/session launching to resolve SDK contextTier from the selected context window, and adjust tests accordingly.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/test/browser/agentSessions/agentHostChatContribution.test.ts	Adds coverage for lifting context size into maxContextWindow and for synthesizing the picker from recommendedContextWindows.
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts	Lifts tokens-group numeric config values into ModelSelection.maxContextWindow and includes it in selection equality checks.
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostLanguageModelProvider.ts	Synthesizes a “Context Size” configuration property from recommendedContextWindows.
src/vs/platform/agentHost/test/node/copilotAgent.test.ts	Updates model schema expectations and adds a materialization test for mapping selected context window → SDK tier.
src/vs/platform/agentHost/test/node/agentSideEffects.test.ts	Updates model fixtures to include recommendedContextWindows.
src/vs/platform/agentHost/node/copilot/copilotSessionLauncher.ts	Moves contextTier derivation into the agent and threads it through launch plans.
src/vs/platform/agentHost/node/copilot/copilotAgent.ts	Exposes recommendedContextWindows, accepts maxContextWindow on selections, and resolves SDK contextTier robustly.
src/vs/platform/agentHost/node/agentSideEffects.ts	Plumbs recommendedContextWindows into side-effect output.
src/vs/platform/agentHost/common/state/protocol/channels-root/state.ts	Extends protocol state types with recommendedContextWindows and ModelSelection.maxContextWindow.
src/vs/platform/agentHost/common/agentService.ts	Extends IAgentModelInfo with recommendedContextWindows.

Copilot's findings

• Files reviewed: 10/10 changed files
• Comments generated: 3

[roblourens]

Looks fine in general