Expand PushRulesType to expose all push rule fields

What does this MR do and why?

Contributes to #509936

Problem

The PushRulesType GraphQL type only exposed reject_unsigned_commits, while the REST API exposes all push rule fields. This limits GraphQL clients from querying the full push rules configuration.

Solution

Add all push rule fields to PushRulesType to achieve parity with the REST API. Feature-gated fields (commit_committer_check, commit_committer_name_check, reject_unsigned_commits, reject_non_dco_commits) return false when unavailable.

References

• Issue: #509936
• REST API reference: ee/lib/api/project_push_rule.rb

How to set up and validate locally

1. Visit: http://gdk.test:3000/-/graphql-explorer
2. Query a project with push rules configured:

   query {
     project(fullPath: "your/project") {
       pushRules {
         denyDeleteTag
         memberCheck
         preventSecrets
         commitMessageRegex
         commitMessageNegativeRegex
         branchNameRegex
         authorEmailRegex
         fileNameRegex
         maxFileSize
         commitCommitterCheck
         commitCommitterNameCheck
         rejectUnsignedCommits
         rejectNonDcoCommits
       }
     }
   }

3. Verify all fields return expected values

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