[rush] Consolidate pnpm options handling into InstallHelpers.resolvePnpmSettings

[iclanton]

Summary

Consolidates how Rush interprets the pnpm options when generating the install files, so the options are read and version-gated in a single place instead of being duplicated across the install managers. This is intended to be a behavior-preserving refactor.

Details

• Adds InstallHelpers.resolvePnpmSettings(...) — the single place that reads a subspace's pnpm options, emits the version-compatibility warnings, and derives both:
  • the pnpm field written to common/temp/package.json, and
  • a PnpmWorkspaceFile (common/temp/pnpm-workspace.yaml) populated with the version-gated settings (overrides, packageExtensions, peerDependencyRules, allowedDeprecatedVersions, patchedDependencies, allowBuilds, catalogs, minimumReleaseAge, etc.).
• generateCommonPackageJsonAsync now just consumes the resolved settings rather than re-reading and re-gating the pnpm options itself. WorkspaceInstallManager and RushInstallManager both route through resolvePnpmSettings, removing the duplicated pnpm 11 gating logic that previously lived in two places.
• Adds a RushConstants.pnpmWorkspaceFileName constant and replaces the hard-coded 'pnpm-workspace.yaml' strings.
• Adds PnpmWorkspaceFile.loadAsync(...), which reads and parses pnpm-workspace.yaml once and returns a populated instance. The rush-pnpm patch-commit/patch-remove and approve-builds commands now use it, replacing the previous per-field loaders / inline YAML parsing.

How it was tested

• heft test for rush-lib (703 tests passing).
• The rush-lib API report (common/reviews/api/rush-lib.api.md) is updated accordingly.