Honor allow_exceptions=false for excessive array/object size (out_of_range.408)

[nlohmann]

Summary

The size sanity check in json_sax_dom_parser/json_sax_dom_callback_parser's start_object()/start_array() (include/nlohmann/detail/input/json_sax.hpp) throws out_of_range.408 directly via JSON_THROW, instead of routing through parse_error() — the only place that honors the SAX consumer's allow_exceptions flag. Every other malformed-input error in the binary readers (truncated input, invalid bytes, bad UTF-8 in CBOR sizes, etc.) correctly yields a discarded value when allow_exceptions is false; this one path didn't, and could also terminate JSON_NOEXCEPTION builds.

json::from_cbor(std::vector<uint8_t>{0x9b, 0x80,0,0,0,0,0,0,0}, true, false);  // CBOR array, length 2^63
// before: throws out_of_range.408, even though allow_exceptions=false
// after:  returns a discarded value, per the documented allow_exceptions contract

Affects CBOR (array and map), UBJSON, and BJData. MessagePack cannot trigger it on 64-bit platforms (lengths are at most 32-bit) and BSON has no element counts, so neither needs a code change.

Fix

Replace the four JSON_THROW(out_of_range::create(408, ...)) call sites with return parse_error(0, "", out_of_range::create(408, ...)), matching the pattern already used by every other error path in these two classes. The binary readers already propagate a false return from sax->start_object()/sax->start_array() correctly (the same mechanism every other error condition uses), so no changes were needed there.

This is behavior-preserving when allow_exceptions == true (still throws the identical exception) — the only behavior change is for allow_exceptions == false.

Test plan

• Added a regression test to tests/src/unit-regression2.cpp covering CBOR array/map, UBJSON, and BJData with an oversized declared length, both with allow_exceptions=false (now is_discarded()) and the default allow_exceptions=true (still throws, message unchanged), plus a truncated-input regression guard.
• Verified the new is_discarded() assertions fail (throw) without the fix and pass with it.
• Regenerated single_include/nlohmann/json.hpp via make amalgamate.

Breaking change?

No breaking changes to the public API. allow_exceptions=true (the default) behavior is byte-for-byte unchanged. allow_exceptions=false now correctly returns a discarded value instead of throwing/aborting for this specific malformed-input case, matching the documented contract for every other malformed-binary-input case — code that relied on the previous (undocumented, inconsistent) throwing behavior under allow_exceptions=false would need to check is_discarded() instead, same as it already must for every other kind of parse error.

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