Support OpenQASM end statements

[danielgaskins]

Summary

• accept OpenQASM 3 end; statements during validation and unrolling
• stop statically unrolled blocks after an unconditional terminator
• preserve conditional terminators inside runtime-dependent branches
• cover global, branch, loop, switch, box, and subroutine behavior

Testing

• pytest -q tests/qasm3/test_end.py
• pytest -q tests
• pytest -q --cov=pyqasm --cov-config=pyproject.toml --cov-report=term --cov-report=xml tests
• pylint src tests bin examples
• isort --check-only src tests bin examples
• black --check --target-version py310 src tests bin examples
• mypy src examples
• qbraid admin headers src tests bin examples --skip=src/pyqasm/_version.py --type=apache
• sphinx-build -W -b html docs/ <output-directory>

Fixes #396

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[github-actions]

👋 Hey there! It looks like the changelog might need an update.

Please take a moment to edit the CHANGELOG.md with:

• A brief, one-to-two sentence summary of your changes.
• A link back to this PR for reference.
• (Optional) A small working example if you've added new features.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.43590% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/pyqasm/visitor.py	97.43%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[TheGupta2012]

Can you use already built test validators in our utilities? It seems like we already have a pretty extensive utility module built for these assertions.

[TheGupta2012]

Why do you need all statements in this method as a parameter? I think we can just isolate the processing inside the analyser.py and refactor

[TheGupta2012]

Hi @danielgaskins , thanks for extending pyqasm with this contribution! I just gave a couple of comments but otherwise looks good. Can you also add a CHANGELOG entry besides these changes?