Fix release script bundler frozen mode and docs build error handling

setup-ruby's `bundler-cache: true` sets frozen mode, which prevents
`bundle install` from updating Gemfile.lock after script/release
bumps the gemspec version. The stale lockfile then makes
`bundle exec rake docs:build` fail with:

  The gemspecs for path gems changed, but the lockfile can't be updated
  because frozen mode is set

Because the script had no error handling on that command, the failure
was silently ignored and the release PR was pushed with unbuilt docs.

Fix both problems:

- Export BUNDLE_FROZEN=false and BUNDLE_DEPLOYMENT=false at the top of
  the script so every bundler invocation can rewrite the lockfile.
- Switch `update_gemfiles` from `bundle` (install) to `bundle lock`,
  matching the appraisal loop — we only need the lockfile refreshed,
  gems are already installed by setup-ruby.
- Fail the script when `sync_contributors.rb` or `rake docs:build`
  returns non-zero, so a broken release cannot be pushed.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 217f0a0d-a9f2-4607-9643-5fcb3e34f20f

Author: joelhawksley

script/release

@@ -1,5 +1,12 @@
 #!/usr/bin/env bash
 
+# bundler-cache: true in setup-ruby enables frozen mode, which blocks
+# updating Gemfile.lock after the gemspec version bump. Disable it for
+# every bundler invocation in this script so the lockfile can be
+# regenerated to match the new version.
+export BUNDLE_FROZEN=false
+export BUNDLE_DEPLOYMENT=false
+
 fetch() {
   git fetch --all
 }
@@ -54,8 +61,10 @@ update_ruby_version() {
 }
 
 update_gemfiles() {
-  # Update Gemfile.lock
-  bundle
+  # Update Gemfile.lock. Use `bundle lock` (not `bundle install`) so we
+  # only rewrite the lockfile with the new version — no gem installs
+  # needed, and it avoids re-triggering the bundler-cache frozen check.
+  bundle lock
 
   # Update appraisal gemfile locks.
   # BUNDLE_IGNORE_RUBY_VERSION is needed because the gemfiles declare ruby
@@ -66,8 +75,8 @@ update_gemfiles() {
 }
 
 build_docs() {
-  ruby script/sync_contributors.rb
-  bundle exec rake docs:build
+  ruby script/sync_contributors.rb || { echo "Error: sync_contributors.rb failed"; exit 1; }
+  bundle exec rake docs:build || { echo "Error: rake docs:build failed"; exit 1; }
 }
 
 add_changed_files() {