test: cover actions permission JSON marshalling

[KirtiRamchandani]

Problem

Issue #55 tracks adding JSON marshal coverage for public resource types that implement String().

Root cause

Several Actions permission-related response types had String() implementations but no matching marshal tests, leaving their JSON field names and omitted-empty behavior uncovered.

Solution

Add marshal tests for the remaining Actions permission and retention structs in the existing test files:

• ArtifactPeriod
• ActionsPermissionsEnterprise
• SelfHostRunnerPermissionsEnterprise
• SelfHostedRunnersSettingsOrganization
• WorkflowsPermissions
• ContributorApprovalPermissions

Tests

• go test ./github -run 'Test(ArtifactPeriod|ActionsPermissionsEnterprise|SelfHostRunnerPermissionsEnterprise|SelfHostedRunnersSettingsOrganization|WorkflowsPermissions|ContributorApprovalPermissions)_Marshal'
• go test ./github
• go test ./...
• git diff --check

Fixes part of #55.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.48%. Comparing base (c207b27) to head (5126bb3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4250   +/-   ##
=======================================
  Coverage   97.48%   97.48%           
=======================================
  Files         190      190           
  Lines       19178    19178           
=======================================
  Hits        18695    18695           
  Misses        268      268           
  Partials      215      215           

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