[python] cudaq.pauli_word is opaque (str() returns the repr), inconsistent with SpinOperatorTerm.get_pauli_word returning str

[wsttiger]

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Describe the feature

A cudaq.pauli_word constructed in Python is opaque: it has no public
attributes or methods (dir() shows none) and str(word) returns the
object repr, so host code cannot inspect, compare, or serialize the word
it wraps. Meanwhile SpinOperatorTerm.get_pauli_word(n) returns a plain
Python str, and plain strings are accepted as list[cudaq.pauli_word]
kernel arguments — so the type only becomes load-bearing (and opaque)
exactly where users must construct it manually (captured values).

import cudaq
w = cudaq.pauli_word("XIZ")
print([m for m in dir(w) if not m.startswith("_")])  # []
print(str(w))
# <cudaq.mlir._mlir_libs._quakeDialects.cudaq_runtime.pauli_word object at 0x...>

Suggestion: give cudaq.pauli_word a __str__ (and ideally
__eq__/__repr__) returning the word string.

Environment

• CUDA-Q built from source at commit 0be565550f4c23affdcbed9e4eaec38d2d0915e6
• Python 3.11.x, target qpp-cpu (library mode)
• AlmaLinux 8.10, x86_64

All reproducers below were executed and verified against this build.