Add unit tests for Issue subtype toString implementations

[ogolberg]

Summary

• Adds dedicated unit tests for each Issue subtype's toString() method in IssueToStringTest
• Covers singular and plural variations for MissingApplicationSuperType, FinalApplicationSuperType, and MissingSuperType (which use the .readable extension)
• Tests the MemberAccess.description() "via" variant when declaring type differs from target type
• Removes the redundant toString assertions from ExpediterIntegrationTest

Test plan

• All 16 new tests pass via ./gradlew :model:test --tests "com.toasttab.expediter.issue.IssueToStringTest"
• ExpediterIntegrationTest still passes after removing the toString block
• spotlessCheck passes for both :model and :tests

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.38%. Comparing base (1b7337a) to head (cceaee7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #233      +/-   ##
============================================
+ Coverage     87.80%   88.38%   +0.58%     
  Complexity      287      287              
============================================
  Files            43       43              
  Lines          1033     1033              
  Branches        155      155              
============================================
+ Hits            907      913       +6     
+ Misses           66       61       -5     
+ Partials         60       59       -1     

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