Describe the bug/error/problem
While testing the fluent-plugin-elasticsearch against elasticsearch >= 9.1.2, we encountered a NoMethodError when calling the _bulk API.

It seems that the update_ndjson_headers! method assumes that the Content-Type header always includes the compatible-with= parameter, which causes a crash if a client explicitly sends a pure application/x-ndjson header.

In lib/elasticsearch/api/utils.rb:

version = client_headers[current_content].match(/compatible-with=([0-9]+)/)[1] || 9

If client_headers[current_content] is exactly "application/x-ndjson" (or "application/json"), the regex .match returns nil. Attempting to call [1] on nil triggers the NoMethodError, completely crashing the request before it is sent.

It was introduced at #2759

To Reproduce

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'
  gem 'elasticsearch', '= 9.1.2'
  # gem 'elasticsearch', '= 9.1.1'
end

require 'elasticsearch'

puts "Elasticsearch Client Version: #{Elasticsearch::VERSION}"
puts "-" * 40

client = Elasticsearch::Client.new(
  url: 'http://localhost:9200',
  transport_options: {
    headers: {
      'Content-Type' => 'application/x-ndjson'
    }
  }
)

client.bulk(body: [
  { index: { _index: 'test_index', _id: '1' } },
  { field: 'value' }
])

• result:

$ ruby sample.rb
Fetching gem metadata from https://rubygems.org/..........
Resolving dependencies...
Elasticsearch Client Version: 9.1.2
----------------------------------------
/home/watson/.rbenv/versions/4.0.5/lib/ruby/gems/4.0.0/gems/elasticsearch-api-9.1.2/lib/elasticsearch/api/utils.rb:185:in 'Elasticsearch::API::Utils#update_ndjson_headers!': undefined method '[]' for nil (NoMethodError)

        version = client_headers[current_content].match(/compatible-with=([0-9]+)/)[1] || 9
                                                                                   ^^^
        from /home/watson/.rbenv/versions/4.0.5/lib/ruby/gems/4.0.0/gems/elasticsearch-api-9.1.2/lib/elasticsearch/api/actions/bulk.rb:190:in 'Elasticsearch::API::Actions#bulk'
        from sample.rb:23:in '<main>'

Or

irb(main):001> ''.match(/compatible-with=([0-9]+)/)[1]
(irb):1:in '<main>': undefined method '[]' for nil (NoMethodError)
        from <internal:kernel>:168:in 'Kernel#loop'
        from /home/watson/.rbenv/versions/3.4.9/lib/ruby/gems/3.4.0/gems/irb-1.18.0/exe/irb:9:in '<top (required)>'
        from /home/watson/.rbenv/versions/3.4.9/bin/irb:25:in 'Kernel#load'
        from /home/watson/.rbenv/versions/3.4.9/bin/irb:25:in '<main>'

Expected behavior
As Question:

Is it strictly mandatory for clients to include compatible-with=X in the headers for NDJSON endpoints (like _bulk and _msearch) starting from 9.1.2?

• If it is an unintended regression: Could we fix this by adding a safe navigation operator? (e.g., match(/.../)&.[](1) || 9)
• If it is strictly required by design: We will update our plugin to explicitly inject the compatible-with parameter to avoid this crash.

Thank you for your time and great work on this gem!

Your Environment (please complete the following information):

• Operating System any
• Ruby Version any
• Elasticsearch client version 9.1.2
• Elasticsearch version 9.4.2
• Any other relevant information

Additional context
Add any other context about the problem here.