Handle missing milestone state in issue events

[subotac]

Description

Handle partial issue-event milestone payloads that omit state. Missing values now return null, while unknown non-null values use GHMilestoneState.UNKNOWN through EnumUtils.

Includes a regression using the existing issue-event WireMock fixture.

Fixes #2028.

REST schema: https://docs.github.com/en/rest/issues/events?apiVersion=2022-11-28#list-issue-events

Before submitting a PR

• Binary backwards compatibility checked with japicmp
• JavaDocs describe the nullable return value
• Added tests covering the changed behavior
• mvn -D enable-ci clean install passes, including SpotBugs and JaCoCo checks
• Local site generation exceeded a 20-minute timeout while downloading its cold reporting dependency graph; all preceding build gates passed
• Changes are on a branch other than main

When creating a PR

• Includes Fixes #2028 and the relevant REST schema link
• Changed lines are covered by tests
• Allow edits from maintainers is enabled

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.41%. Comparing base (79e0fc6) to head (5ee500d).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #2316   +/-   ##
=========================================
  Coverage     85.41%   85.41%           
  Complexity     2568     2568           
=========================================
  Files           242      242           
  Lines          7576     7577    +1     
  Branches        399      399           
=========================================
+ Hits           6471     6472    +1     
  Misses          869      869           
  Partials        236      236           

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