Refactor: move part of updating protected branch logic to service layer

[lunny]

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PR Overview

This PR refactors the protected branch update logic by moving part of it to the service layer, centralizing functionality and reducing redundancy. Key changes include:

• Introducing a new service function (CreateOrUpdateProtectedBranch) in services/pull/protected_branch.go to encapsulate the update and PR recheck logic.
• Updating the web and API route handlers to call the new service function instead of duplicating the branch-check logic.
• Removing redundant inline recheck logic and cleaning up obsolete comments.

Reviewed Changes

File	Description
services/pull/protected_branch.go	New service function for creating or updating protected branches.
routers/web/repo/setting/protected_branch.go	Updated to use the new service function for protected branch updates.
routers/api/v1/repo/branch.go	Removed duplicate branch-check logic in favor of the new service.
services/forms/repo_form.go	Removed extraneous ASCII art comments.

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services/pull/protected_branch.go:22

• [nitpick] Consider renaming 'isPlainRule' to 'isRegularRule' for improved clarity on the branch rule type.

isPlainRule := !git_model.IsRuleNameSpecial(protectBranch.RuleName)