feat!: Support reason in UsersService.Unsuspend

[JamBalaya56562]

Fixes #4613.

BREAKING CHANGE: UsersService.Unsuspend now takes an UnsuspendUserRequest body argument.

The GHES Unsuspend a user endpoint (DELETE /users/{username}/suspended) accepts an optional reason that is recorded in the audit log, but Unsuspend always sent no body. This adds UnsuspendUserRequest{Reason *string} and passes it by value, matching Suspend. Pass UnsuspendUserRequest{} to unsuspend without a reason.

The new type follows the naming and value passing used for Suspend in #4612; the two PRs touch different parts of users_administration.go and can be merged in either order.

Verified locally: go build ./..., go vet -tags integration ./test/integration/, example module build, go test ./..., generated files idempotent, custom-gcl 0 issues, and script/check-schema-fields.sh reports no new issues.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.61%. Comparing base (e754d2c) to head (047621a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4614   +/-   ##
=======================================
  Coverage   98.61%   98.61%           
=======================================
  Files         199      199           
  Lines       18693    18693           
=======================================
  Hits        18435    18435           
  Misses        258      258           

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[gmlewis]

Thank you, @JamBalaya56562!
LGTM.

cc: @stevehipwell - @Not-Dhananjay-Mishra

[Not-Dhananjay-Mishra]

Could you please add a comment documenting its default value?

The reason the user is being unsuspended. This message will be logged in the audit log. If you don't provide a reason, it will default to "Unsuspended via API by SITE_ADMINISTRATOR", where SITE_ADMINISTRATOR is the person who performed the action.