chore: Add Dependabot alert marshal tests

[KirtiRamchandani]

Problem

• Issue add tests for resource JSON marshalling #55 tracks missing JSON marshal test coverage for exported structs.
• The Dependabot alert models covered service behavior, but AdvisoryEPSS, DependabotAlert, and DependabotAlertState did not have direct marshal round-trip tests.

Root cause

• These structs were added/expanded without corresponding testJSONMarshal coverage.

Solution

• Add JSON marshal/unmarshal round-trip tests for AdvisoryEPSS, DependabotAlert, and DependabotAlertState.
• Cover nested advisory, vulnerability, dismissal, timestamp, user, and repository fields on DependabotAlert.

Tests run

• gofmt -w github/dependabot_alerts_test.go
• go test ./github -run 'Test(AdvisoryEPSS|DependabotAlert|DependabotAlertState)_Marshal'
• go test ./github
• go test ./...
• git diff --check

Updates #55

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.48%. Comparing base (c207b27) to head (cf662e0).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4251   +/-   ##
=======================================
  Coverage   97.48%   97.48%           
=======================================
  Files         190      190           
  Lines       19178    19178           
=======================================
  Hits        18695    18695           
  Misses        268      268           
  Partials      215      215           

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[gmlewis]

Thank you, @KirtiRamchandani!
LGTM.
Awaiting second LGTM+Approval from any other contributor to this repo before merging.

cc: @stevehipwell - @alexandear - @zyfy29 - @Not-Dhananjay-Mishra - @munlicode