test(decode): add coverage for expected helper and unreachable diagnostic paths

[harningt]

Resolves coverage gaps in lua/json/decode.lua and lua/json/decode/util.lua:

• Expected Helper Coverage: Added test_util_expected_helper and test_util_expected_single_helper tests to cover both paths of the expected(...) function in util.lua.
• Diagnostic Decoder Cleanup: Removed the dead if not parsed and return state.previous checks in diagnostic_decoder (which always matches/fails without returning).
• Decoder Assertion Guard: Added an assert(false) guard at the end of the diagnostic_decoder loop noting that it is unreachable under normal runtime conditions.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.97%. Comparing base (e883555) to head (2e4b56b).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
lua/json/decode.lua	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #55      +/-   ##
===========================================
+ Coverage    95.42%   95.97%   +0.55%     
===========================================
  Files           18       18              
  Lines         1071     1069       -2     
===========================================
+ Hits          1022     1026       +4     
+ Misses          49       43       -6     

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