Make a shallow copy of metadata in QuantumCircuit <> DAG conversion

[matteoacrossi]

Fixes #10570. Fixes #16140.

As discussed in #10570, metadata is only assigned in circuit_to_dag and dag_to_circuit, resulting in potential side effects when modifying the metadata of one of the circuit instances.

This PR implements the change proposed in #10570 to dag_to_circuit, making a shallow copy of the metadata field.
For circuit_to_dag the change is made in converters.rs, I hope it is the right place.

It also adds two quick tests.

AI/LLM disclosure

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[jakelishman]

Thanks for the PR. I left a few comments about why I'm not sure we can unilaterally make this change in #16140 (comment) - I think we're going to need to take a bit more care with API stability, because people could easily be relying on the current behaviour.

It's probably best to keep the conversation over there for the time being, then pivot this PR?

[jakelishman]

This place (Rust-space extraction of the fields from a Python-space object) is most likely not the right place to put this - that's going to cause the copy in the wrong places, I think. It so happens that circuit_to_dag calls this right now (I think?), but that's just one of the users.