Slack notification service links incorrectly formatted when the link text contains square brackets

Summary

GitLab CI job names can contain pairs of square brackets ([ ]) thanks to the parallel:matrix feature. When link text contains a pair of square brackets, the links are not formatted correctly by the Slack notification service.

Steps to reproduce

1. Apply this patch (git apply patch-file)

   diff --git a/spec/models/integrations/chat_message/pipeline_message_spec.rb b/spec/models/integrations/chat_message/pipeline_message_spec.rb
   index a80d13d7f5d..fa3be2b29c9 100644
   --- a/spec/models/integrations/chat_message/pipeline_message_spec.rb
   +++ b/spec/models/integrations/chat_message/pipeline_message_spec.rb
   @@ -332,6 +332,22 @@
       end
     end
   
   +  context "when name contains brackets" do
   +    before do
   +      args[:builds] = [
   +        { id: 1, name: "test: [arg1, arg2]", status: "failed", stage: "test" }
   +      ]
   +    end
   +
   +    it "correctly formats links" do
   +      expect(subject.attachments.first[:fields][3]).to eq(
   +        title: "Failed jobs",
   +        value: "<http://example.gitlab.com/-/jobs/1|test: [arg1, arg2]>",
   +        short: true
   +      )
   +    end
   +  end
   +
     context "when the CI config file contains a YAML error" do
       let(:has_yaml_errors) { true }

2. Run bundle exec rspec spec/models/integrations/chat_message/pipeline_message_spec.rb

3. Tests fail when they should pass

   Failures:
   
     1) Integrations::ChatMessage::PipelineMessage when name contains brackets correctly formats links
       Failure/Error:
         expect(subject.attachments.first[:fields][3]).to eq(
           title: "Failed jobs",
           value: "<http://example.gitlab.com/-/jobs/1|test: [arg1, arg2]>",
           short: true
         )
       
         expected: {:short=>true, :title=>"Failed jobs", :value=>"<http://example.gitlab.com/-/jobs/1|test: [arg1, arg2]>"}
               got: {:short=>true, :title=>"Failed job", :value=>"[test: [arg1, arg2]](http://example.gitlab.com/-/jobs/1)"}
       
         (compared using ==)
       
         Diff:
         @@ -1,4 +1,4 @@
           :short => true,
         -:title => "Failed jobs",
         -:value => "<http://example.gitlab.com/-/jobs/1|test: [arg1, arg2]>",
         +:title => "Failed job",
         +:value => "[test: [arg1, arg2]](http://example.gitlab.com/-/jobs/1)",
         
       # ./spec/models/integrations/chat_message/pipeline_message_spec.rb:343:in `block (3 levels) in <top (required)>'
       # ./spec/spec_helper.rb:406:in `block (3 levels) in <top (required)>'
       # ./spec/support/sidekiq_middleware.rb:9:in `with_sidekiq_server_middleware'
       # ./spec/spec_helper.rb:397:in `block (2 levels) in <top (required)>'
       # ./spec/spec_helper.rb:393:in `block (3 levels) in <top (required)>'
       # ./lib/gitlab/application_context.rb:31:in `with_raw_context'
       # ./spec/spec_helper.rb:393:in `block (2 levels) in <top (required)>'
       # ./spec/support/database/query_analyzer.rb:9:in `block (3 levels) in <main>'
       # ./lib/gitlab/database/query_analyzer.rb:42:in `within'
       # ./spec/support/database/query_analyzer.rb:9:in `block (2 levels) in <main>'
       # ./spec/support/database/prevent_cross_joins.rb:102:in `block (3 levels) in <main>'
       # ./spec/support/database/prevent_cross_joins.rb:56:in `with_cross_joins_prevented'
       # ./spec/support/database/prevent_cross_joins.rb:102:in `block (2 levels) in <main>'
   
   Finished in 3 minutes 9.7 seconds (files took 1 minute 27.35 seconds to load)
   38 examples, 1 failure
   
   Failed examples:
   
   rspec ./spec/models/integrations/chat_message/pipeline_message_spec.rb:342 # Integrations::ChatMessage::PipelineMessage when name contains brackets correctly formats links

Branch example: https://gitlab.com/gitlab-org/gitlab/-/commits/bwill/slack-brackets-links-example

What is the current bug behavior?

The links are presented in markdown format [text](link)

What is the expected correct behavior?

The links should be presented in Slack link format <link|text>

Possible fixes

We use ::Slack::Messenger::Util::LinkFormatter in order to format these links. In many places in the codebase, we have a link_text and a link which we format into a markdown string like [link_text](link) and then pass it to LinkFormatter#format. This seems very inefficient, since LinkFormatter#format needs to parse out the link_text and link using regex. Instead, we should probable make LinkFormatter#slack_link public, and give it the link_text and link.