Stop using CGI.parse due to support Ruby 3.5

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
At Ruby 3.5, CGI.parse will be removed.
https://bugs.ruby-lang.org/issues/21258

Therefore, the test fails where CGI.parse is used.

[master][~/src/fluentd]$ ruby -v -I"lib:test" test/plugin_helper/http_server/test_request.rb -v
ruby 3.5.0dev (2025-05-17T00:42:12Z master aa0f689bf4) +PRISM [x86_64-linux]
/home/watson/.rbenv/versions/3.5-dev/lib/ruby/gems/3.5.0+1/gems/async-pool-0.10.3/lib/async/pool/controller.rb:331: warning: assigned but unused variable - error
/home/watson/.rbenv/versions/3.5-dev/lib/ruby/gems/3.5.0+1/gems/traces-0.15.2/lib/traces/config.rb:44: warning: assigned but unused variable - error
/home/watson/src/fluentd/lib/fluent/plugin_helper/http_server/request.rb:17: warning: CGI library is removed from Ruby 3.5. Please use cgi/escape instead for CGI.escape and CGI.unescape features.
If you need to use the full features of CGI library, Please install cgi gem.
/home/watson/src/fluentd/lib/fluent/plugin_helper.rb:46: warning: method redefined; discarding old inherited
/home/watson/src/fluentd/lib/fluent/plugin_helper.rb:46: warning: previous definition of inherited was here
Loaded suite test/plugin_helper/http_server/test_request
Started
HttpHelperRequestTest:
  test_request:                                                                                                                 E
================================================================================================================================================
Error: test_request(HttpHelperRequestTest): NoMethodError: undefined method 'parse' for class CGI
/home/watson/src/fluentd/lib/fluent/plugin_helper/http_server/request.rb:38:in 'Fluent::PluginHelper::HttpServer::Request#query'
test/plugin_helper/http_server/test_request.rb:14:in 'HttpHelperRequestTest#test_request'
     11:
     12:     assert_equal('/path', request.path)
     13:     assert_equal('foo=42', request.query_string)
  => 14:     assert_equal({'foo'=>['42']}, request.query)
     15:     assert_equal('text/html', request.headers['content-type'])
     16:     assert_equal(['gzip'], request.headers['content-encoding'])
     17:   end
================================================================================================================================================
: (0.005199)

Finished in 0.005372395 seconds.
------------------------------------------------------------------------------------------------------------------------------------------------
1 tests, 2 assertions, 0 failures, 1 errors, 0 pendings, 0 omissions, 0 notifications
0% passed
------------------------------------------------------------------------------------------------------------------------------------------------
186.14 tests/s, 372.27 assertions/s

This patch will replace CGI.parse with URI.decode_www_form

Docs Changes:
Not needed.

Release Note:

• in_monitor_agent: stop using CGI.parse due to support Ruby 3.5
• HTTP server plugin helper: stop using CGI.parse due to support Ruby 3.5

[daipom]

Note: About compatibility of Request#query of Http Server Plugin Helper.

This fix maintains the compatibility. (Thanks!)

On the other hand, if we could abandon the compatibility, then it would be simpler and more useful.

def query
  @query_string && Hash[URI.decode_www_form(@query_string)]
end

Before: "a=foo&b=bar" => {"a"=>["foo"], "b"=>["bar"]}
After: "a=foo&b=bar" => {"a"=>"foo", "b"=>"bar"}

This would be more natural as a parsed result.
In addition, we can remove the annoying code of in_monitor_agent that is taking values by using Hash.new { |_, _| [] } and .first.

As a plugin helper, it would be very important to maintain compatibility, but the current specification seems inconvenient, so it might be good if it could be improved (maybe in the future).

[daipom]

LGTM! Thanks!