fix(jsonc): report unterminated strings as unexpected end of input

[tomas-zijdemans]

An unterminated string now reports the truncation instead of blaming the token.

Before, parse('"abc') threw:

Cannot parse JSONC: unexpected token ""abc" in JSONC at position 0

Every other truncated input says Cannot parse JSONC: unexpected end of JSONC input. Unterminated strings were the exception, and {"a": "b had the same problem.

The tokenizer's string branch yields a String token whether or not it found the closing quote. The failure then surfaces downstream, when JSON.parse chokes on the token text, so it reads as a bad token rather than a file that stopped early. The fix tracks hasEndOfString and throws the end-of-input error, mirroring the hasEndOfComment check for unterminated block comments 50 lines above it.

Still a SyntaxError. Message only, no type change, no change to any input that parses.

7 lines in parse.ts, plus 2 cases in the existing parse() throws error message test. #5799 aligned the other jsonc messages the same way.

deno fmt, deno lint, deno task lint:docs, and the jsonc suite (408 tests) all pass.

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 95.03%. Comparing base (ca58f94) to head (58789ba).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7302      +/-   ##
==========================================
- Coverage   95.03%   95.03%   -0.01%     
==========================================
  Files         617      618       +1     
  Lines       51637    51898     +261     
  Branches     9359     9410      +51     
==========================================
+ Hits        49075    49323     +248     
- Misses       2021     2030       +9     
- Partials      541      545       +4     

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