[Backport #2008 to 5.x] Make AbstractAnnotation::$_context non-nullable

[github-actions]

Automated backport of #2008 to branch 5.x.

Do not modify this pull request.

Closes #2007

Summary

• Change AbstractAnnotation::$_context type from ?Context to Context
• The constructor guarantees a non-null Context across all three initialization branches
• The nullable type was a leftover from the pre-native-type PHPDoc @var Context|null, converted in Code cleanup #1947

Test plan

• Full test suite passes (1037 tests)
• CI matrix

[github-actions]

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin backport-2008-to-5.x
git worktree add --checkout .worktree/backport-2008-to-5.x backport-2008-to-5.x
cd .worktree/backport-2008-to-5.x
git reset --hard HEAD^
git cherry-pick -x ede7ae7d989edb272f73172d5c33e1476b8dba26