fix(amplify-cli-core): detect packageManager field in package.json hierarchy

[danrivett]

Problem

The getPackageManager() function in amplify-cli-core doesn't detect the packageManager field in package.json, which is the standard corepack convention for specifying which package manager a project uses (e.g., "packageManager": "pnpm@10.17.0").

This causes failures in two scenarios:

1. Corepack enforcement (Node.js 24+, where corepack is enabled by default): Amplify detects the wrong package manager based on lock files, then fails when corepack rejects the invocation. For example, running amplify build in a pnpm monorepo with "packageManager": "pnpm@10.x" produces:

   🛑 InvalidOverrideError: Received error [Error: Command failed with exit code 1: yarn --version
   This project is configured to use pnpm because <project-root>/package.json has a "packageManager" field]
   

2. pnpm workspaces with no local lock file: When the Amplify project is in a subdirectory and no lock file is present locally (the lock file lives at the workspace root outside the Amplify directory), Amplify falls back to whichever package manager executable it finds on PATH (typically yarn), ignoring the packageManager field that specifies pnpm.

These failures will become increasingly common as corepack adoption grows and Node.js 24+ enables it by default.

Solution

Adds findPackageManagerFieldInHierarchy() that walks up the directory tree looking for a packageManager field in package.json files, mimicking corepack's own resolution behavior. When found, it takes precedence over lock file detection. If the specified package manager executable isn't found in PATH, it gracefully falls back to the existing detection logic.

Updated detection order

1. Check if package.json exists (mandatory)
2. Check for packageManager field in package.json hierarchy (new)
3. Check if pnpm-lock.yaml is present + pnpm executable
4. Check if yarn.lock is present + yarn executable
5. Check if package-lock.json is present
6. Check if yarn is in PATH
7. Fallback to npm

Issue #, if available

Fixes #13931
Addresses #12001

Description of how you validated changes

• 6 new unit tests added covering:
  • Detect pnpm from packageManager field in package.json
  • Detect npm from packageManager field in package.json
  • Detect yarn from packageManager field in package.json
  • Detect packageManager field from parent directory in hierarchy
  • packageManager field takes precedence over lock files
  • Falls back to lock file detection when packageManager executable not found
• Full amplify-cli-core test suite passes (21 suites, 198 tests)
• Manual testing with pnpm monorepo using corepack and "packageManager": "pnpm@10.x" in root package.json

Checklist

• If this is a bug fix, the change doesn't introduce a breaking API change
• If this is a new feature/enhancement, it was first discussed and approved in a GitHub issue
• The change is covered by automated tests
• The change passes all existing tests

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.