[rush] Add PNPM global virtual store support via RUSH_PNPM_ENABLE_GLOBAL_VIRTUAL_STORE

[Jérémy Dieuze (QuanticPotatoes)]

Summary

Follow-up to #5830 (currently conflicts with main). Original work by EscapeB (@EscapeB); this PR rebases it onto current main and keeps the env-var design from that review.

When RUSH_PNPM_ENABLE_GLOBAL_VIRTUAL_STORE=1, workspace installs set enableGlobalVirtualStore: true in the generated pnpm-workspace.yaml. Package instances live in the shared PNPM store instead of being recreated under each worktree's common/temp/node_modules/.pnpm.

The lockfile does not change. The package-manager install-lock skip stays in #5844.

Details

• Opt-in via RUSH_PNPM_ENABLE_GLOBAL_VIRTUAL_STORE, not pnpm-config.json. A per-subspace config flag would allow mixed layouts in one repo (called out on [rush] Add PNPM global virtual store support for rush install #5830).
• Requires a shared store: pnpmStore global, or RUSH_PNPM_STORE_PATH pointing outside the repo. A worktree-local store throws. A path override inside the repo warns and still enables the flag (store stays local, so no worktree win).
• Requires PNPM >= 10.12.1.
• Incompatible with usePnpmSyncForInjectedDependencies (pnpm-sync still assumes node_modules/.pnpm in the worktree).
• Last-install flag records the setting so flipping it forces a reinstall.
• Adapted to the current PnpmWorkspaceFile / EnvironmentConfiguration shape (public workspace fields, saveAsync, module-level env state, globalPnpmfile).

How it was tested

• Existing unit tests from [rush] Add PNPM global virtual store support for rush install #5830, updated for the current APIs (PnpmWorkspaceFile.test.ts, WorkspaceInstallManager.test.ts, BaseInstallManager.test.ts, EnvironmentConfiguration.test.ts, LastInstallFlag.test.ts).
• rush-package-manager-integration-test coverage from [rush] Add PNPM global virtual store support for rush install #5830 kept.
• Dogfood on a 7-subspace Rush repo that already uses RUSH_PNPM_STORE_PATH (shared content store). The four installed subspaces still keep a worktree-local virtual store: 5.4 GB / 531k inodes under node_modules/.pnpm (api alone: 2.0 GB / 204k inodes / 2606 instances). With this PR and RUSH_PNPM_ENABLE_GLOBAL_VIRTUAL_STORE=1, rush install --subspace api wrote enableGlobalVirtualStore: true. Worktree common/temp/api/node_modules/.pnpm dropped to 1.5 MB / 2354 inodes (2 top-level entries). Package instances moved to ~/.rush-pnpm-store/v10/links (+2.0 GB). A second worktree on the same store installed in 6.2s; store size did not grow; .pnpm stayed 1.5 MB. Lockfile unchanged.

EscapeB (@EscapeB), happy to close this if you already have a rebase in flight. Otherwise this is meant to land #5830.

Fixes #5830

[Jérémy Dieuze (QuanticPotatoes)]

Rebased onto current main (was CONFLICTING).

Conflict resolution:

• TestHelper.ts: kept the inode-identity check for workspace links (needed when GVS hardlinks) and fs.realpath for the error paths, matching the main-side helper change.
• rush-lib.api.md: restored trimRushEnvironmentVariablesForOperations that the rebase auto-merge dropped.

Local verification on the rebased tip:

• rush test --to @microsoft/rush-lib: 948 tests, 0 failures (API Extractor included)
• PNPM 10.12.1 GVS integration test: enable, switch off without --purge, switch back, lockfile unchanged, build OK