Update release-npm workflow for dryruns

[rekhoff]

This PR addresses the npm dry-run failures observed in recent release-dry-run CI runs, where pnpm publish --dry-run contacts the npm registry and errors out for already-published versions like spacetimedb@2.7.1.

Description of Changes

Updates tools/release/src/targets/npm.rs so that in dry-run mode the npm release target:

1. runs pnpm run prepublishOnly to execute the build/test/size checks
2. runs pnpm pack to create the tarball locally

The live pnpm publish --no-git-checks path is unchanged. This also makes the log and error messages accurate for whichever mode is running.

API and ABI breaking changes

No changes.

Expected complexity level and risk

Low. Only the dry-run branch of tools/release/src/targets/npm.rs is changed. The live npm publish path still runs pnpm publish.

Testing

• cargo clippy -p spacetimedb-release passes.
• The dry-run path should be exercised by running cargo-release release npm v2.7.1 --dry-run or the release-dry-run workflow on a branch.

[bfops]

I didn't realize you had already started a PR for this. Closing in favor of the merged #5636