fix: typeScript compilation error and test failures in pnpm support

[Copilot]

Addresses build and test failures preventing pnpm support from merging.

Changes

• Fixed TypeScript error in PackageManagerService.isPackageInstalled(): Changed const args to let args to allow reassignment in pnpm case, which constructs a different argument structure than npm/yarn
• Fixed test expecting no package manager detection: Test now removes all lock files before asserting error is thrown
• Added missing package-lock.json in react test fixtures: File existed in core but was absent in react, causing test failures

Example

// Before: compilation error
const args: string[] = [packageManager, "list", "--depth=1", "--json"];
// ...
case PackageManagerType.pnpm:
  args = [packageManager, "list", packageName, "--json", "--depth=1"]; // Error: Cannot assign to const

// After: compiles correctly
let args: string[] = [packageManager, "list", "--depth=1", "--json"];
// ...
case PackageManagerType.pnpm:
  args = [packageManager, "list", packageName, "--json", "--depth=1"]; // OK

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