fix: parse first complete unfenced JSON value

[fritz-fujii]

Summary

Fix parse_json_markdown() so multiple unfenced JSON values do not get concatenated into a single json.loads() call.

The parser now consumes the first complete unfenced JSON value while preserving the existing fenced-content and scalar behavior.

Problem

Given:

{"a": 1}
{"a": 2}

the current implementation selects the first opening brace and the last closing brace, causing both values to be passed to json.loads() and raising JSONDecodeError: Extra data.

This surfaces in the Question Classifier node when a model emits more than one JSON value in a row.

Fix

Use json.JSONDecoder().raw_decode() for the unfenced JSON path so parsing stops after the first complete JSON value.

Existing fenced JSON behavior is intentionally preserved — in particular, test_parse_and_check_json_markdown_multiple_blocks_fails (multiple fenced blocks still fail) is left unchanged.

Tests

Added regression coverage for multiple unfenced JSON values and verified the existing JSON-in-Markdown parser tests continue to pass.

New cases: first value is an object / an array / contains nested objects / contains brackets inside string values / is followed by prose, a malformed first value still raises, and the reported parse_and_check_json_markdown failure mode now resolves.

tests/unit_tests/libs/test_json_in_md_parser.py: 20 passed (13 existing + 7 new).

Checklist

• This change requires a documentation update, included: Dify Document — not needed, internal parser behavior only
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly. — N/A
• I ran make lint && make type-check (backend) and vp staged (frontend) to appease the lint gods

Note on the last item: the full make targets need uv, which was not available in my environment. I ran the equivalent checks scoped to the two changed files instead: ruff check (all checks passed), ruff format --check (already formatted), and mypy --check-untyped-defs (no issues). Happy to run anything else the full pipeline flags.

Fixes #42006

From Claude Code

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	63.92%	63.91%	-0.01%
Strict coverage	63.52%	63.51%	-0.01%
Typed symbols	45,606	45,593	-13
Untyped symbols	25,904	25,911	+7
Modules	3384	3384	0