chat: Add experiment-backed MCP authentication hints setting - #336484

Summary

Recurring proactive MCP authentication reminders can interrupt otherwise unrelated conversations. Add a way to suppress only these reminders so we can evaluate their effect on retention without disabling MCP or bypassing authentication.

• Add application-scoped chat.mcp.authenticationHints.enabled, defaulting to true, with experiment: { mode: 'auto' }. The shared registration covers editor chat and the Agents Window.
• Gate new proactive reminders and update existing reminders when the setting changes. Suppression does not mark a server as authenticated or a reminder as actioned.
• Preserve silent authentication, manual sign-in, authentication prompts blocking actual tool calls, and progress for user-initiated authentication already in flight.
• Record chat.mcp.authenticationHintEligible once per open conversation in both variants. Its only event-specific property is the effective hintsEnabled boolean; it records no server names, URLs, credentials, or conversation content.

Experiment / rollout

Treatment variable: config.chat.mcp.authenticationHints.enabled

• Control: true (existing behavior)
• Treatment: false (suppress proactive reminders)

Explicit user settings take precedence over experiment defaults. The eligibility event supports identifying the affected population in either arm instead of selecting only users who saw a reminder. Review the new telemetry event before rollout, and target only builds containing this change. This PR does not create or enable a remote experiment and does not change the default experience.

Validation

Rebased onto current main (de1a430a2f3).

• npm run gulp compile-client — passed, including client type-checking.
• Hygiene and lint on all six changed files — passed.
• 16 focused Electron unit tests — passed, covering default/enabled/disabled behavior, eligibility deduplication, live configuration changes, preserved silent and tool-call authentication, and existing reminder rendering.

./scripts/test.sh \
  --run src/vs/workbench/contrib/chat/test/browser/agentSessions/agentHostChatContribution.test.ts \
  --run src/vs/workbench/contrib/chat/test/browser/widget/chatContentParts/chatMcpAuthenticationContentPart.test.ts \
  --run src/vs/workbench/contrib/chat/test/browser/agentSessions/stateToProgressAdapter.test.ts \
  --grep 'mcp auth prompt|ChatMcpAuthenticationContentPart|authentication-required invocation'

Try locally

Set "chat.mcp.authenticationHints.enabled": false to suppress reminders. Toggle it while a reminder is visible to verify that hiding it does not authenticate the server, and that authentication required by an actual tool call is still actionable.

To exercise experiment assignment instead, leave the setting unset and use:

"experiments.override": {
  "config.chat.mcp.authenticationHints.enabled": false
}