Use extended diagnostics in parse errors

[nlohmann]

The extended diagnostics could also help to improve the parse error exceptions. They could provide, in addition to line/column/byte count a path the last parsed value.

Example:

{
    "foo": [1, 2, flase]
}

The illegal literal flase could be easier to find when we provide a JSON Pointer /foo or even /foo/2.

For reference, this is the current exception:

[json.exception.parse_error.101] parse error at line 2, column 20: syntax error while parsing value - invalid literal; last read: '2, fl'

[nlohmann]

JSON_DIAGNOSTICS currently augments type_error/out_of_range-style exceptions with a JSON pointer to the offending value, but parse errors (like the invalid literal example given) happen before any value exists to point at, so this specific enhancement — attaching the JSON pointer of the last successfully parsed value — is not implemented. It's a plausible, debugging-friendly improvement, though it would require the parser to track pointer-path state during the parse rather than only at exception-throw time, which is a bit more invasive than the existing diagnostics feature.

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This reply was drafted by Claude Code on behalf of @nlohmann, as part of a review of open discussions.