test: add coverage for bypass_modes in rulesets

[oikarinen]

Add test coverage for all three bypass_modes (always, pull_request, exempt) for both repository and organization rulesets.

Repository Ruleset Tests (resource_github_repository_ruleset_test.go):

• Add test for all three bypass_modes with Team actors
• Add test for updating bypass_mode (always → exempt)
• Add test for different actor types with different bypass_modes (Team/always, RepositoryRole/pull_request, OrganizationAdmin/exempt)

Organization Ruleset Tests (resource_github_organization_ruleset_test.go):

• Fix existing "Creates and updates organization using bypasses" test:
  • Move bypass_actors from incorrect location (inside rules) to correct location (at ruleset level)
  • Add missing bypass_mode assertions
  • Fix incorrect actor_type assertions (was checking for "0", "5" instead of actual actor type strings)
  • Correct OrganizationAdmin actor_id from 0 to 1
• Add test for all three bypass_modes
• Add test for updating bypass_mode

All tests now verify:

• actor_id (both dynamic Team IDs and static role IDs)
• actor_type (Team, RepositoryRole, OrganizationAdmin, DeployKey)
• bypass_mode (always, pull_request, exempt)

This addresses the review comment from PR #2764 requesting test coverage for bypass_modes, including the newly added "exempt" mode.

Fixes: #2764 (review)

Fixes #2755

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Before the change?

• no coverage for any of the bypass_modes for rulesets

After the change?

• tests!

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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