Fix serializer crash when multi_json is loaded without RubyGems activation

[jwils]

Problem

Since 8.5.2, Serializer::MultiJson#deprecated_gem_version_loaded? chooses between the deprecated MultiJson API and the new MultiJSON API (multi_json >= 1.21.0) by consulting:

Gem.loaded_specs['multi_json'].version < Gem::Version.create('1.21.0')

Gem.loaded_specs is only populated when gems are activated through RubyGems. When gems are loaded from a plain $LOAD_PATH — e.g. a bundle install --standalone bundle, or a vendored load path — the lookup returns nil, and every request fails during response deserialization:

NoMethodError: undefined method 'version' for nil
  .../lib/elastic/transport/transport/serializer/multi_json.rb:60:in 'deprecated_gem_version_loaded?'
  .../lib/elastic/transport/transport/serializer/multi_json.rb:40:in 'load'
  .../lib/elastic/transport/transport/base.rb:362:in 'perform_request'

8.5.1 and earlier were unaffected (the serializer called ::MultiJson directly, without consulting gem specs). We hit this in ElasticGraph's CI, which runs part of its test suite from a standalone bundle, and pinned to 8.5.1 to work around it.

Reproduction

$ mkdir repro && cd repro
$ printf 'source "https://rubygems.org"\ngem "elastic-transport", "8.5.3"\n' > Gemfile
$ bundle install --standalone
$ ruby -rbundle/bundler/setup -relastic/transport -e \
    'p Elastic::Transport::Transport::Serializer::MultiJson.new.load("{}")'
.../serializer/multi_json.rb:60:in 'deprecated_gem_version_loaded?':
undefined method 'version' for nil (NoMethodError)

Fix

Detect which multi_json API is available by checking for the MultiJSON constant instead. multi_json 1.21.0 is exactly the version that introduced that constant, so the check is behaviorally equivalent to the version comparison — and it works regardless of how the gem was loaded.

Testing

• Updated the existing serializer unit test to branch on the same constant check.
• Added a regression test that stubs Gem.loaded_specs to be empty; it fails with the production NoMethodError before this change and passes after.
• Manually verified round-trip serialization with both multi_json 1.15.0 (deprecated MultiJson API) and 1.21.1 (MultiJSON API) loaded from a plain $LOAD_PATH without RubyGems activation.

One note from running rake test:unit locally (macOS, Ruby 3.4.7): the connection_test.rb resurrect tests fail intermittently both with and without this change (they appear to be ordering/seed-sensitive), while the serializer tests pass consistently.

Related: #125 is a different symptom in the same version-detection code path; this PR doesn't address it.