Add coverage for Copilot CLI node-pty shimming

[anthonykim1]

Resolves: #314128

This adds targeted coverage for the runtime node-pty shim regression from #313609 / #313550. The goal is to make PR checks catch the practical VSIX/Marketplace failure mode that blocked Copilot CLI shell commands in 1.118.x.

Coverage added:

• CopilotCLISDK.ensureShims() is exercised through the real production path before SDK import.
• The test uses a synthetic Marketplace-style layout by separating the Copilot extension extensionPath from VS Code appRoot.
• Runtime shim materialization is asserted for both ripgrep and node-pty under the SDK expected locations.
• copyNodePtyFiles() directly verifies pty.node and spawn-helper land in node_modules/@github/copilot/sdk/prebuilds/<platform>-<arch>.
• The existing shims.txt skip behavior is covered without changing marker semantics.

This intentionally stays lighter than installing an actual VSIX in CI: it runs in the existing Copilot Vitest/unit PR checks (Copilot - Test) while still covering the runtime copy path that broke the marketplace-installed extension route.

[github-actions]

Base: ffb86233 Current: 7cdb4c92

No screenshot changes.

[Copilot]

Pull request overview

Adds unit test coverage in the Copilot CLI node test suite to guard against regressions in runtime shimming for ripgrep and node-pty (tracked in #314128), ensuring marketplace install layouts keep working.

Changes:

• Adds a new Vitest spec that sets up a synthetic VS Code appRoot + extensionPath tree and asserts ripgrep/node-pty files are copied into the SDK’s expected shim locations.
• Adds a test case to verify shim creation is skipped when shims.txt already exists.

Show a summary per file

File	Description
extensions/copilot/src/extension/chatSessions/copilotcli/node/test/copilotCliShims.spec.ts	New unit tests covering Copilot CLI shim materialization + shims.txt skip behavior.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 2