change: json_decode rejects malformed JSON instead of silent partial (#495)

[InauguralPhysicist]

The recursive-descent JSON parser was lenient: arrays/objects break on an unexpected token and returned the partial container, json_decode never checked for trailing input, and numbers stopped at the first non-numeric char. So malformed input succeeded silently — and a genuine JSON null was indistinguishable from a parse failure (both returned null).

Now raises value (was a silent partial)

Input	Was	Now
{ , {]	{}	raises
[1,2, , [1,2,3	[1, 2] / [1, 2, 3]	raises
{"a": , {"a":1,}	{"a": null} / {"a": 1}	raises
[1,2,3] trailing	[1, 2, 3]	raises
42abc	42	raises
"" , notjson	null	raises
unterminated string, >200 deep	partial / truncated	raises

Valid JSON is unchanged, and a bare null still decodes to null — now distinguishable from a parse failure.

Design

A thread-local strict-parse error flag is set by the parse functions on any malformed condition (unexpected token, truncation, unterminated string, over-deep nesting). builtin_json_decode resets it on entry and, after parsing one value, requires only whitespace remains — raising value otherwise. The flag is checked only by json_decode: json_path and the ext_http header/body parsers call the same parser but ignore it, keeping their historical lenient behavior.

Load-bearing check

Nothing tested json_decode == null for validity, and every stdlib/test/example feeds valid JSON. Zero suite tests broke except test_json_depth — a >200-deep document now raises catchably rather than returning a truncated value; the security property ("no SIGSEGV") is preserved and re-asserted.

Tests

Suite section [116] extended — test_raise_on_bad_input.eigs, now 43 checks (truncated/partial/garbage raise; valid arrays/objects and a bare null still parse).

• Release suite: 2668/2668
• Rebased onto current main (with for-in uses live list length: append during iteration never terminates #491); ASan + detect_leaks=1: 2677/2677, leak tally 0 (partial-container error path is leak-clean).

Remaining from the audit

• parser: missing expression after is/of silently becomes null (x is, print of, throw of) #494 parser: missing expr after is/of → null (expression parser)
• docs: LANGUAGE_CONTRACT + GRAMMAR still document pre-#405 / pre-#406 semantics as Enforced #492 docs drift (LANGUAGE_CONTRACT/GRAMMAR)

Closes #495

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