Add barrier label rendering with barrier_label_len to the Rust circuit drawer

[kmbys]

Part of #16464 (do not auto-close: this is one item of the tracking issue).

Summary

The Rust circuit drawer previously did not render Barrier labels at all.
This PR ports the barrier label rendering of the Python text drawer,
including the barrier_label_len truncation behavior added in #15776:

• The label is drawn above the top-most wire of the barrier; the other
  wires keep the plain ░ column, matching the Python text drawer.
• Labels longer than barrier_label_len characters are truncated at that
  length with a ... suffix.
• draw_circuit gains a barrier_label_len argument, and the C API's
  QkCircuitDrawerConfig struct gains a barrier_label_len field.
  0 selects the default of 16 characters, so existing struct
  initializers that omit the field keep the previous behavior.

Details and comments

• OnWireElement::Barrier now carries a reference to its instruction so
  the label is available at render time.
• The rendering was validated character-for-character against released
  qiskit==2.5.0 output, in both merged and unmerged wire modes.
• New unit tests cover: default truncation (16), a custom truncation
  length, a barrier whose top-most wire is not the first wire of the
  circuit, and a label of exactly 16 characters (not truncated).
• A features_c release note is included for the new
  QkCircuitDrawerConfig field.

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: Claude Fable 5
• I used the following tool to generate or modify code: Claude Fable 5

[qiskit-bot]

Thank you for opening a new pull request.

Before your PR can be merged it will first need to pass continuous integration tests and be reviewed. Sometimes the review process can be slow, so please be patient.

While you're waiting, please feel free to review other open PRs. While only a subset of people are authorized to approve pull requests for merging, everyone is encouraged to review open pull requests. Doing reviews helps reduce the burden on the core team and helps make the project's code better for everyone.

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[CLAassistant]

All committers have signed the CLA.