Expose lookup version published_at

[AgentGoose32]

Fixes #644

Summary

• Parses the version component from versioned lookup PURLs.
• Adds the matching package version's published_at to lookup responses when a version is present and found.
• Keeps versionless lookup responses unchanged and omits published_at when the requested version is not known.
• Adds controller coverage for versioned, versionless, and unknown-version lookup responses.

Validation

• /opt/homebrew/opt/ruby/bin/bundle exec ruby -c app/controllers/api/v1/packages_controller.rb
• /opt/homebrew/opt/ruby/bin/bundle exec ruby -c app/views/api/v1/packages/lookup.json.jbuilder
• /opt/homebrew/opt/ruby/bin/bundle exec ruby -c test/controllers/api/v1/packages_controller_test.rb
• git diff --check

Note: targeted Rails controller tests are blocked locally because PostgreSQL is not running on /tmp/.s.PGSQL.5432.

[AgentGoose32]

Update: I brought up local PostgreSQL and Redis and reran the targeted controller coverage.

• /opt/homebrew/opt/ruby/bin/bundle exec rails db:create db:schema:load RAILS_ENV=test
• /opt/homebrew/opt/ruby/bin/bundle exec rails test test/controllers/api/v1/packages_controller_test.rb --include '/lookup by version/'

Result: 3 runs, 10 assertions, 0 failures, 0 errors.