Qiskit / qiskit-ibm-runtime / 32725774601

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keep only the zero point for selected extrap (#3166)

### Summary
Return the selected extrapolators for each observable term only for the
zero noise extrapolation point.

### Details and comments

Fixes one of the issues raised in #3144 .
There is no use in the selected extrapolations for the other
extrapolation points, as the we return expectation value calculation for
each extrapolator type, and not a calculation of mixed extrapolator
types (like we do for the zero noise point).
The data is still not returned to the user, and should be added
somewhere in the metadata (if decided to return this information at
all).

### AI/LLM disclosure

- [X] I didn't use LLM tooling, or only used it privately.
- [ ] I used the following tool to help write this PR description:
- [ ] I used the following tool to generate or modify code:

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