Fix missing fractional gates in primitive in session

[nkanazawa1989]

Summary

We cannot run primitive inside session with fractional gates. This PR fixes the bug.

Details and comments

This is the current behavior:

from qiskit_ibm_runtime import QiskitRuntimeService, Session, SamplerV2

service = QiskitRuntimeService()
backend = service.backend(use_fractional_gates=True)

with Session(backend=backend) as _:
    primitive = SamplerV2()
    assert primitive._backend.options.use_fractional_gates is True  # fail

Because of this, this session fails in running the primitives because of the ISA circuit validation. When we instantiate a primitive inside the session context, the backend is reloaded from the service attached to the session without the optin flag.