Allow knip updates and remove the redundant Firestore dev dependency

[cristianrgreco]

Summary

Two small follow-ups to #1492, one commit each:

1. knip uses a caret range (^6.39.0) like the other dev dependencies, instead of an exact version. The lockfile still installs 6.39.0.
2. The gcloud module no longer declares @google-cloud/firestore as a dev dependency, and the knip ignore for it is removed. With that, knip.jsonc has no dependency ignores left.

Why remove the Firestore declaration

The Firestore test imports firebase-admin/firestore and never imports @google-cloud/firestore directly. firebase-admin loads that package at runtime and already brings it in as an optional dependency. So the module's own declaration was a second, exact-pinned copy of the same requirement, and knip reported it as unused.

Without it:

• The package is still installed, at the same version (9.2.0). In the lockfile it and 24 packages under it are now marked optional, because firebase-admin's optional dependency is the only thing requiring them.
• firebase-admin chooses a Firestore version it supports. Before, two exact pins (firebase-admin 14.5.0 and @google-cloud/firestore 9.2.0) had to be kept in step by hand.
• The Firestore test now relies on optional dependencies being installed, which npm ci does by default.

Verification

• npx vitest run packages/modules/gcloud/src/firestore-emulator-container.test.ts with Docker, after the removal: 1 of 1 passed.
• npm ci succeeds with the new lockfile. npm ls shows knip@6.39.0 and @google-cloud/firestore@9.2.0 under firebase-admin.
• npm run knip with configuration hints treated as errors: passes.
• npm run format, npm run lint:ci, npm run check-compiles: pass.
• Lockfile diff against main: the root manifest's knip range, the gcloud workspace's manifest entry, and the optional flag on those 25 packages. No versions change.

Why this isn't breaking

Dev dependencies and tooling config only. @testcontainers/gcloud's published dependencies don't change, and no package source changes.