fix(stream): close thinking block before text or tool output

If upstream omits reasoning-end and starts emitting text-delta or
tool-call events, the thinking block remained open causing
thinking_start -> thinking_delta -> text_start -> ... -> thinking_end
ordering. Now endThinking() is called at the start of text-delta and
tool-call handlers so thinking_end always precedes text_start or
toolcall_start.

Adds 2 regression tests for the missing reasoning-end edge case.