Fixes #327468

What changed

• Registers github.copilot.chat.workspace.prototypeAdoCodeSearchEndpointOverride as application-scoped and restricted.
• Uses the existing userScopeOnly configuration mechanism for the current and migrated legacy keys, so raw workspace and multi-root folder values cannot affect the credential-bearing ADO search request path.
• Adds regression coverage for workspace, workspace-folder, current user, and legacy user values, plus manifest metadata.

Validation

• npm run precommit
• npm run typecheck (Copilot extension)
• npm run compile (Copilot extension)
• npm run test:unit -- src/platform/configuration/vscode/test/configurationServiceImpl.spec.ts src/extension/test/node/configurations.spec.ts — 10 passed
• Targeted ESLint for all changed TypeScript files
• Full Copilot unit suite with four workers: 9,551 passed; four unrelated tests timed out at 5 seconds in the constrained local runner, and none involved the changed configuration paths.

The extension-wide npm run lint exceeded the local automation's 10-minute command limit without emitting diagnostics; targeted lint completed successfully.