Managed Settings: add smoketest coverage

[rwoll]

Establish a migration safety net proving standard VS Code policy, the legacy
Copilot-policy bridge, and direct managed settings produce the expected
behavior and compose restrictively.

Problem

VS Code has focused tests for policy and managed-settings components, but no
product smoke test proving that standard VS Code policy, legacy Copilot policy,
and Enterprise Managed Settings reach the expected client/runtime behavior.

Proposed coverage

Case	Path	Expected result
Standard non-chat policy	VS Code policy service	A conflicting user setting is overridden, locked, and visible in Policy Diagnostics
Legacy Copilot policy	VS Code policy → compatibility bridge → SDK/runtime	Shell approval and domain restrictions are enforced
Managed settings	Managed-settings resolution → SDK/runtime	Equivalent shell and domain restrictions are enforced
Mixed migration case	Legacy policy + managed settings	Sources compose toward the more restrictive result

Checklist

• Add an isolated Policy Plumbing suite under test/smoke.
• Cover one safe non-chat policy such as ExtensionsAutoUpdate="off".
• Run one deterministic shell/domain scenario through the legacy policy path.
• Run the same scenario through equivalent managed settings.
• Assert Policy Diagnostics reports the expected source in each case.
• Assert denied operations never execute and the agent session remains usable.
• Add the mixed-source composition case after the parity cases are stable.
• Isolate policy state and COPILOT_CACHE_HOME between variants.

Implementation pointers

• Existing full-product agent flows:
  test/smoke/src/areas/agentsWindow/agentsWindow.test.ts
• Smoke environment and runtime-state isolation: test/smoke/src/utils.ts
• Legacy policy translation:
  src/vs/platform/agentHost/common/agentHostManagedSettings.ts
• Managed-settings test setup:
  .github/skills/policy-and-managed-settings/local-testing.md
• Policy Diagnostics:
  src/vs/workbench/browser/actions/developerActions.ts

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