* fix(thinking): add deepseek-v4 to thinking model patterns
deepseek-v4-flash emits reasoning_content via the API but was not
recognized in _THINKING_MODEL_PATTERNS (only deepseek-r1 and
deepseek-reasoner were listed). Add the deepseek-v4 prefix so
the model is recognized as thinking-capable.
The between-round _thinkOpen leakage was separately fixed by
PR #5931 (perf(chat): batch live thinking rendering).
Related: #3998, #5931
* test(thinking): cover DeepSeek v4 detection
---------
Co-authored-by: Alexandre Teixeira <alexandremagteixeira@gmail.com>
Co-authored-by: Alexandre Teixeira <111787685+alteixeira20@users.noreply.github.com>
