Add ProjectPushRulesUpdate GraphQL mutation

What does this MR do and why?

Contributes to #509936

Problem

There was no GraphQL mutation to update push rules for a project. Users had to use the REST API to modify push rules configuration.

Solution

Add ProjectPushRulesUpdate mutation that allows updating all push rule fields via GraphQL. The mutation uses the existing PushRules::CreateOrUpdateService for upsert logic and supports all fields including feature-gated ones (commit_committer_check, reject_unsigned_commits, etc.).

References

• Issue: #509936
• Depends on: !223061 (merged) (Expand PushRulesType to expose all push rule fields)

Screenshots or screen recordings

N/A - No UI changes

How to set up and validate locally

1. Start a Rails console or GraphiQL
2. Run the mutation on a project where you have maintainer access:

   mutation {
     projectPushRulesUpdate(input: {
       fullPath: "your/project"
       denyDeleteTag: true
       preventSecrets: true
       commitMessageRegex: "^(feat|fix|docs):"
       maxFileSize: 100
     }) {
       pushRules {
         denyDeleteTag
         preventSecrets
         commitMessageRegex
         maxFileSize
       }
       errors
     }
   }

3. Verify the push rules are updated and returned correctly

MR acceptance checklist

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