Qiskit / qiskit-ibm-runtime / 36590869562

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Ensure that `dag.unit` does not emit non-actionable warning. (#3391)

### Summary
Currently, the scheduling transpiler passes get and set the `unit`
attribute of dag circuits, which is deprecated and will be removed in
Qiskit 3.0. Unfortunately, there does not seem to be an obvious solution
to this.

The scheduling passes in qiskit-ibm-runtime are inspired, in large part,
by Qiskit scheduling passes. Last year, Qiskit [PR
#14133](https://github.com/Qiskit/qiskit/pull/14133) replaced the call
to `unit` with a call to `_unit`
([link](https://github.com/Qiskit/qiskit/blame/main/qiskit/transpiler/passes/scheduling/padding/base_padding.py#L132)).
The PR mentions that "these deprecation warnings are raised to the user
as if they were user-actionable (which they are not). [...] flooding
test runs and confusing both users and downstream package devs."

This PR follows a similar strategy. In particular:
- It introduces a helper that's in charge for calling `unit`, so that
every call to `unit` is centralized to a single place.
- It replaces `unit` with `_unit`, adding an inline comment to make sure
there's enough context for the future.

With this change, the tests in `unit/transpiler` go down from 1085 to
153.

### 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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