Fix unwanted phase in QROAMClean

[rupasri118]

Summary

Fixes #1968.

QROAMClean used approximate CSwap gates in its swap network, which can introduce unwanted relative phases. This caused the output state to have a negative amplitude for an address that should have a positive amplitude.

This change uses exact CSwap gates in QROAMClean to prevent unwanted phases while preserving approximate CSwap as the default behavior for SwapWithZero.

Changes

• Use exact CSwap gates in QROAMClean.
• Preserve approximate CSwap as the default for SwapWithZero.
• Add a regression test to verify that the output state has the correct positive amplitude.
• Update T-complexity expectations to account for the exact CSwap implementation.

Testing

• qroam_clean_test.py — 26 passed, 1 skipped, 1 xfailed.
• qroam_clean_test.py, swap_with_zero_test.py, and cswap_approx_test.py — 79 passed, 6 skipped, 3 xfailed.

[rupasri118]

Hi @mhucka, could you please take a look at this PR when you get a chance? It fixes the unwanted phase issue in QROAMClean (#1968). I've updated the branch and checked the relevant tests. I'd really appreciate your feedback. Thank you!