chore: Add attestation marshal tests

[KirtiRamchandani]

Problem
Artifact attestation payload types are shared by organization, repository, and user attestation APIs, but the shared resource structs did not have dedicated JSON marshal coverage tracked by #55.

Root cause
Existing service tests exercise API decoding paths, while the resource-level marshal round-trip tests skipped Attestation and AttestationsResponse.

Solution
Add marshal tests for empty and populated Attestation and AttestationsResponse values. The populated cases use the existing json.RawMessage comparator so Sigstore bundle JSON is compared semantically.

Tests run

• gofmt -w github/attestations_test.go
• go test ./github -run 'TestAttestation(sResponse)?_Marshal'
• go test ./github
• go test ./...
• git diff --cached --check

Linked issue
Part of #55

[gmlewis]

Suggested change

	// Copyright 2024 The go-github AUTHORS. All rights reserved.
	// Copyright 2026 The go-github AUTHORS. All rights reserved.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.48%. Comparing base (c207b27) to head (8613564).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4253   +/-   ##
=======================================
  Coverage   97.48%   97.48%           
=======================================
  Files         190      190           
  Lines       19178    19178           
=======================================
  Hits        18695    18695           
  Misses        268      268           
  Partials      215      215           

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[KirtiRamchandani]

Thanks, updated the new test file copyright year to 2026 and reran the focused marshal test plus diff check.