fix: preserve assert_return generic type

[simPod]

Summary

• fix the assert_return phpdoc generic contract so static analysis preserves TValue
• keep the runtime behavior unchanged while making phpstan infer the returned asserted type

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.93%. Comparing base (c810866) to head (3bb0c86).
⚠️ Report is 1 commits behind head on 0.2.x.

Additional details and impacted files

@@           Coverage Diff           @@
##            0.2.x      #84   +/-   ##
=======================================
  Coverage   85.93%   85.93%           
=======================================
  Files           6        6           
  Lines          64       64           
=======================================
  Hits           55       55           
  Misses          9        9           

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