test(jsonc): make the JSONTestSuite harness compare parsed values

[tomas-zijdemans]

The JSONTestSuite harness has never compared a parsed value. This makes it compare them.

getError(() => { JSON.parse(text); }) uses a block body with no return, so jsonResult and jsoncResult come back undefined every time. The assertEquals(jsonResult, jsoncResult) on line 47 has been comparing undefined to undefined since it was written. The test_transform/ corpus exists for exactly this check (number precision, duplicate keys, NFC/NFD keys, invalid codepoints) and none of it was running.

Switching to expression bodies returns the values.

Second fix: walk() had no extension filter, so README.md and test.ts were registered as JSON fixtures. Both parsers reject them, so they "passed". Added exts: [".json"], which is why the count drops from 408 to 406.

Does this catch a real bug? No. Everything still passes, so nothing was hiding. To confirm the assertion is live rather than merely present, I sabotaged the jsonc result and watched 159 tests fail, the 159 fixtures that parse successfully under both parsers. That same sabotage passed clean before this change.

No production code touched.

One follow-up I left out on purpose: the harness only checks whether something threw, never the type. That hides two fixtures, n_structure_100000_opening_arrays.json and n_structure_open_array_object.json, where jsonc throws RangeError and JSON.parse throws SyntaxError. Asserting the error type means first deciding what jsonc owes on recursion depth, so it gets its own issue rather than riding along here.

I used Claude Code to help investigate and write this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.04%. Comparing base (ca58f94) to head (3afe9d6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7303   +/-   ##
=======================================
  Coverage   95.03%   95.04%           
=======================================
  Files         617      618    +1     
  Lines       51637    51640    +3     
  Branches     9359     9359           
=======================================
+ Hits        49075    49080    +5     
+ Misses       2021     2019    -2     
  Partials      541      541           

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