chore: Add dependency graph marshal tests

[KirtiRamchandani]

Problem
Dependency graph and SBOM resource structs are newer API payloads, but their JSON marshal behavior was not covered by the resource-level marshal tests tracked by #55.

Root cause
The existing dependency graph tests focus on service request/response behavior. They do not round-trip the shared SBOM, dependency snapshot, manifest, detector, job, and creation response structs directly.

Solution
Add focused marshal tests for empty and populated dependency graph resource types, including SBOM metadata, package external refs, relationships, snapshot manifests, resolved dependencies, and snapshot creation data.

Tests run

• gofmt -w github/dependency_graph_marshal_test.go
• go test ./github -run 'Test(SBOM|CreationInfo|RepoDependencies|PackageExternalRef|SBOMRelationship|SBOMInfo|DependencyGraphSnapshot.*)_Marshal'
• go test ./github
• go test ./...
• git diff --cached --check

Linked issue
Part of #55

[gmlewis]

Instead of creating a new file for this, please delete this file and add the tests to the end of the already-existing dependency_graph_test.go.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.48%. Comparing base (c207b27) to head (c393feb).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4254   +/-   ##
=======================================
  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, moved the marshal tests into the existing dependency_graph_test.go file and removed the standalone test file. Reran the focused marshal test group, go test ./github, go test ./..., and git diff --check.