reasoning_tokens=None has been carrying two meanings at once, no
reasoning and no report, and the library resolved the ambiguity by
quietly claiming the first. ThinkingObservation splits them: UNKNOWN
says the call left no signal, ABSENT says the provider reported zero.
The verdict ranks evidence by hardness. Reasoning prose is the fact
itself; reasoning_tokens is a report about the fact, so a missing
report cannot overrule prose that is right there. The prose check
strips first, since a gateway that returns whitespace is not evidence.
The enum lives in types.py, not in the new thinking.py, because
LLMResponse is typed on it and the innermost layer must not import a
decision module.
