feat: add Fast Walsh–Hadamard Transform for Pauli-Liouville conversion

[H33T589]

This PR introduces a new helper function _to_pauli_liouville_fht that uses the Fast Walsh–Hadamard Transform to convert a Liouville superoperator to its Pauli‑Liouville representation in O(N log N) time. The function applies the transform along rows and columns and optionally normalizes the result. No existing functionality is removed; the new helper can be called by existing conversion routines for improved performance.

Resolves or addresses issue #1632 (Fast conversion to and from Pauli basis). Adds 46 lines of code.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 4.34783% with 22 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 98.92%. Comparing base (61472bc) to head (4983b6c). :warning: Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...qibo/quantum_info/superoperator_transformations.py	4.34%	22 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1736      +/-   ##
==========================================
- Coverage   99.09%   98.92%   -0.17%     
==========================================
  Files          80       80              
  Lines       13083    13106      +23     
==========================================
+ Hits        12964    12965       +1     
- Misses        119      141      +22     

Flag	Coverage Δ
unittests	98.92% <4.34%> (-0.17%)	:arrow_down:

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