fix(rush-lib): suppress --prefer-frozen-lockfile during --lockfile-only phase (pnpm v10)

[Copilot]

Bug Fix

What was the bug?

Enabling both usePnpmPreferFrozenLockfileForRushUpdate and usePnpmLockfileOnlyThenFrozenLockfileForRushUpdate together silently breaks rush update on pnpm v10. The two-phase install's Phase 1 was receiving --prefer-frozen-lockfile --lockfile-only simultaneously. In pnpm v10's v9 lockfile format (packages:/snapshots: split), this combination causes pnpm to write a lockfile containing only importers:, omitting packages: and snapshots: entirely. Phase 2 (--frozen-lockfile) then immediately fails:

ERR_PNPM_LOCKFILE_MISSING_DEPENDENCY  Broken lockfile: no entry for '<package>@<version>' in pnpm-lock.yaml

How did you fix it?

In BaseInstallManager.pushConfigurationArgs, add !onlyShrinkwrap to the usePnpmPreferFrozenLockfileForRushUpdate guard so that --prefer-frozen-lockfile is never emitted during the Phase 1 lockfile-write step:

- } else if (experiments.usePnpmPreferFrozenLockfileForRushUpdate) {
+ } else if (experiments.usePnpmPreferFrozenLockfileForRushUpdate && !onlyShrinkwrap) {

This falls through to --no-prefer-frozen-lockfile for Phase 1, ensuring a full resolution and a complete lockfile. --prefer-frozen-lockfile continues to apply for single-phase rush update.

Scenario	Phase 1 flags	Phase 2 flags
Two-phase rush update (both experiments on)	--no-prefer-frozen-lockfile --lockfile-only	--frozen-lockfile
Single-phase rush update	--prefer-frozen-lockfile	n/a
rush install	--frozen-lockfile	n/a

Testing

Added a test to BaseInstallManager.test.ts asserting that when usePnpmPreferFrozenLockfileForRushUpdate is enabled, --prefer-frozen-lockfile is absent from the args when onlyShrinkwrap: true, and present when onlyShrinkwrap: false.