Fix operator @ quantumstate multiplication

[aaryav-3]

Summary

This PR fixes GitHub issue #14726 by ensuring that matrix multiplication using the @ operator on quantum states does not cause problems with output dimensions or data types.

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Checklist

• I have added the tests to cover my changes.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.

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Details and Comments

This change introduces a method implementing the __rmatmul__ magic function within QuantumState, allowing these operations to be handled separately and preventing unexpected type or dimension issues.

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[Cryoris]

Thanks for having a look, I left some comments below.

[Cryoris]

The argument front is not handled right now. I would suggest we just raise an error if this is True and the input is a Statevector.

If $\langle\psi| O$ style multiplication is desired in the future we can still add it, but given that we don't really have a representation of a bra vector I wouldn't open this can of worms here.

[aaryav-3]

Thank you for your comments. I’ve handled this with an exception, and since thedot()method sets front=True, I updated group.py to handle the statevector dot product as a special case. I’d love to hear your opinion on this approach.
Apologies for the delayed reply — I had drafted this earlier but only just realized I hadn’t sent it.

[coveralls]

Pull Request Test Coverage Report for Build 17762403317

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 8 of 8 (100.0%) changed or added relevant lines in 2 files are covered.
• 14 unchanged lines in 5 files lost coverage.
• Overall coverage increased (+0.004%) to 88.292%

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Files with Coverage Reduction	New Missed Lines	%
crates/circuit/src/parameter/parameter_expression.rs	1	82.79%
crates/qasm2/src/expr.rs	1	93.63%
crates/transpiler/src/passes/gate_direction.rs	1	96.3%
crates/qasm2/src/lex.rs	5	91.75%
crates/qasm2/src/parse.rs	6	96.62%

Totals
Change from base Build 17755812814:	0.004%
Covered Lines:	92738
Relevant Lines:	105036

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💛 - Coveralls