(lambda-nodejs): nodeModules with pnpm doesn't inherit project .npmrc for private registry authentication

### Describe the bug

When using `nodeModules` with pnpm, CDK creates a `pnpm-workspace.yaml` in the bundling temp directory (added in [#21911](https://github.com/aws/aws-cdk/pull/21911) to fix [#21910](https://github.com/aws/aws-cdk/issues/21910)). This causes pnpm to treat the bundling directory as a standalone workspace root, which prevents it from inheriting `.npmrc` configuration from parent directories. This breaks authentication for packages from private registries (e.g., AWS CodeArtifact).

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Last Known Working CDK Library Version

_No response_

### Expected Behavior

When `nodeModules` is specified, `pnpm install` in the bundling directory should be able to authenticate with private registries configured in the project's `.npmrc`.

### Current Behavior

`pnpm install` fails with 401 Unauthorized errors when trying to fetch packages (or their dependencies) from private registries, because the bundling directory's `pnpm-workspace.yaml` prevents pnpm from finding the project's `.npmrc` with auth tokens.

### Reproduction Steps

1. Create a pnpm monorepo with a private registry configured in `.npmrc`:

```
@myorg:registry=https://my-private-registry.example.com/npm/
//my-private-registry.example.com/npm/:_authToken=
```

2. Add a `NodejsFunction` with `nodeModules` referencing a package from the private registry:

```
new NodejsFunction(this, 'MyFunction', {
entry: 'src/handler.ts',
bundling: {
nodeModules: ['@myorg/my-private-package'],
},
});
```

3. Run `cdk synth`

4. Observe 401 errors during bundling because pnpm can't find the auth token

### Possible Solution

- Copy the project's `.npmrc` to the bundling directory alongside `pnpm-workspace.yaml`
- Or document this limitation and the workaround (use global `~/.npmrc` or `commandHooks.beforeInstall` to copy `.npmrc`)

### Additional Information/Context

Related: [#21910](https://github.com/aws/aws-cdk/issues/21910), [#21911](https://github.com/aws/aws-cdk/pull/21911)

Workarounds:
- Store auth tokens in global `~/.npmrc` instead of project `.npmrc`
- Use `commandHooks.beforeInstall` to copy `.npmrc` to the bundling directory

### AWS CDK Library version (aws-cdk-lib)

2.194.0

### AWS CDK CLI version

2.1015.0

### Node.js Version

24

### OS

MacOS 15.7.3 (24G419)

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Start by tracing the lambda-nodejs nodeModules bundling flow and where pnpm-workspace.yaml is created. Reproduce the issue with a pnpm monorepo, a private registry, and cdk synth; the work is done when pnpm install can authenticate using the project .npmrc, or the limitation and workaround are documented.

Written by the indexing model from the issue text.