Add IQPE benchmark

[kpassito]

Description

This PR adds an iqpe benchmark for Iterative Quantum Phase Estimation.

The circuit uses one reusable measurement qubit and num_qubits - 1 target qubits. The num_bits parameter controls the number of measured phase bits.

Fixes #797

Tests

I added coverage for the IQPE circuit structure, invalid parameters, num_bits scaling, num_qubits scaling, and invalid qubit counts.

Local checks:

• python -m pytest -o addopts='' tests/test_bench.py: 203 passed
• ruff check src/mqt/bench/benchmarks/iqpe.py tests/test_bench.py: passed
• ruff format --check src/mqt/bench/benchmarks/iqpe.py tests/test_bench.py: passed
• git diff --check: passed

AI disclosure

Codex assisted with implementation and local testing. I reviewed the changes and take responsibility for the submitted code.

The commits include the required footer:

Assisted-by: GPT-5 Codex via Codex

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

If PR contains AI-assisted content:

• I have disclosed the use of AI tools in the PR description.
• AI-assisted commits include an Assisted-by: [Model Name] via [Tool Name] footer.
• I confirm that I have personally reviewed and understood all AI-generated content, and accept full responsibility for it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[flowerthrower]

Dear @kpassito, thanks for the initial draft. Currently this looks like heavily AI generated. Please review our AI guidelines. I left a first quick review that you might want to address personally.

[flowerthrower]

why constrain to 2 qubits only? should be possible for larger sizes too

[flowerthrower]

this should not be the dynamic size of the benchmark (it should be num_qubits)

[kpassito]

Thanks @flowerthrower, I went through the PR again and addressed the review points.

I removed the extra benchmark selection docs section, updated iqpe so num_qubits controls the circuit width, and fixed the changelog entry with the PR number and author.

I also reran the local checks listed in the PR description.

[flowerthrower]

Dear @kpassito, thank you for your continued efforts. Please give us some time to review. We are currently internally discussing the best way forward for this Benchmark.