Qiskit / qiskit-ibm-runtime / 33643471363

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Avoid caching calibration specific backend configurations (#3287)

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### Summary

prevent calibration specific backend data from getting into default
config by
- updating `QiskitRuntimeService._create_backend_obj()` so
configurations retrieved via `calibration_id` remain scoped to request
and are not stored in `_backend_confics` cache
- preserve existing caching behavior for default backend configurations
and fractional-gate selection
- add coverage for:
    - default -> calibrated -> default retrieval
    - calibrated -> default retrieval with an initially empty cache
    - independence of previously created `IBMBackend` instances
    - repeated calibrated requests remaining fresh
    - default configuration caching remaining effective

add a bug-fix release note.

### Details and comments

calibration specific configs are intentionally not cached (follows
`RuntimeClient.backend_configuration()`)

Fixes #

https://github.com/Qiskit/qiskit-ibm-runtime/issues/3275

### AI/LLM disclosure

- [ ] I didn't use LLM tooling, or only used it privately.
- [ ] I used the following tool to help write this PR description:
- [x] I used the following tool to generate or modify code: (Qwen 3.8
27b running locally was used to review final diff)

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9266 of 10258 relevant lines covered (90.33%)

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