fix(deps): add multi_json dependency to 8 legacy REST gems

[torreypayne]

Description

In 8 older, unregenerated REST client libraries (cloudshell_v1alpha1, appengine_v1beta4, appengine_v1beta5, youtube_partner_v1, youtube_analytics_v1, genomics_v1, genomics_v1alpha2, analyticsdata_v1alpha), the runtime constraint for google-apis-core is set to ~> 0.1 (>= 0.1.0, < 1.0.0).

This PR fixes a dormant CI load error in these 8 gems by explicitly adding gem.add_runtime_dependency "multi_json", "~> 1.0" to their .gemspec files while preserving the original google-apis-core ~> 0.1 constraint (no 1.x major version bump).

See workflow run for an example of the CI error.

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💥 Reproducible Error Sample (Without This Fix)

When running tests (bundle install && toys _nested spec or require "google/apis/cloudshell_v1alpha1") in any of these 8 legacy directories without multi_json in the bundle, RSpec/Bundler crashes with the following error:

  1) Google::Apis::CloudshellV1alpha1 should load
     Failure/Error:
       expect do
         require "google/apis/cloudshell_v1alpha1"
       end.not_to raise_error
     
       expected no Exception, got #<Gem::LoadError: multi_json is not part of the bundle. Add it to your Gemfile.> with backtrace:
         # /usr/local/google/home/torreypayne/.gem/ruby/3.2.0/gems/representable-3.2.0/lib/representable/json.rb:1:in `<top (required)>\'
         # /usr/local/google/home/torreypayne/.gem/ruby/3.2.0/gems/google-apis-core-0.18.0/lib/google/apis/core/json_representation.rb:15:in `<top (required)>\'
         # ./lib/google/apis/cloudshell_v1alpha1/service.rb:16:in `<top (required)>\'
         # ./lib/google/apis/cloudshell_v1alpha1.rb:15:in `<top (required)>\'
         # ./spec/generated_spec.rb:21:in `block (3 levels) in <top (required)>\'
         # ./spec/generated_spec.rb:20:in `block (2 levels) in <top (required)>\'

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🔍 Why This Error Occurs

1. Why google-apis-core-0.18.0 is loaded:
   The legacy library specifies gem.add_runtime_dependency "google-apis-core", "~> 0.1", which restricts google-apis-core to the 0.x major version series (resolving 0.18.0).
2. Why representable-3.2.0 is loaded:
   google-apis-core (0.18.0) requires representable (~> 3.0), which resolves the latest 3.x version (representable 3.2.0).
3. Why it crashes (multi_json is not part of the bundle):
   representable 3.2.0 calls require "multi_json". However, because google-apis-core 0.18.0 predates PR #26612 (which added an explicit runtime dependency on multi_json to google-apis-core 1.2.1+), neither google-apis-core (0.18.0) nor representable (3.2.0) declares a gem dependency on multi_json.
   When Bundler loads the gem (require "bundler/setup"), it blocks unlisted gems, raising Gem::LoadError: multi_json is not part of the bundle.

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🛠️ Why This Fix Is Minimal & Safe

• Zero Major Version Bump: Instead of bumping google-apis-core to 1.x (>= 0.15.0, < 2.a), we preserve ~> 0.1 so downstream consumers experience no major version bump.
• Targeted Fix: We simply add gem.add_runtime_dependency "multi_json", "~> 1.0" directly to the 8 legacy .gemspec files, ensuring multi_json is present in the bundle when representable 3.2.0 loads.