extmod/modjson: Detect unterminated composite entities.

[agatti]

Summary

This PR makes the JSON parser raise an exception when handling objects or arrays whose declaration is incomplete, as in missing the closing marker (brace or bracket) and if the missing marker would have been the last non-whitespace character in the incoming string.

Since CPython's JSON parser would raise an exception in such a case, unlike MicroPython's, this PR aligns MicroPython's behaviour with CPython.

This commit fixes issue #17141.

Testing

This was tested with the Unix port, passing all JSON-related tests (existing and new).

Trade-offs and Alternatives

There may be some code out there that relies on this peculiar behaviour, and thus this PR may introduce an incompatibility across MicroPython versions.

On the other hand, the code wouldn't have worked in CPython in the first place as an incomplete JSON object is not meant to be parseable by json.load or json.loads anyway since they expect a fully formed JSON object to be available (either from a string or from a file descriptor).

[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +8 +0.002% PYBV10
     mimxrt:    +8 +0.002% TEENSY40
        rp2:    +8 +0.001% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +2 +0.000% VIRT_RV32

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.54%. Comparing base (7a55cb6) to head (9ef16b4).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #17148      +/-   ##
==========================================
- Coverage   98.54%   98.54%   -0.01%     
==========================================
  Files         169      169              
  Lines       21897    21898       +1     
==========================================
  Hits        21579    21579              
- Misses        318      319       +1     

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[dpgeorge]

Thanks for updating, now merged.