ci: add ruby platform to Gemfile.lock for forced source builds

BUNDLE_FORCE_RUBY_PLATFORM=true (set in the test workflow so sqlite3
compiles from source on Ruby 3.5 preview) requires the lockfile to
include the `ruby` platform. Without it bundle install exits with
"your bundle only supports platforms [...] but your local platform is
ruby".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: nficano

Gemfile.lock

@@ -162,6 +162,7 @@ PLATFORMS
   arm-linux-gnu
   arm-linux-musl
   arm64-darwin
+  ruby
   x86-linux-gnu
   x86-linux-musl
   x86_64-darwin