Add pauli() accessor to PauliProductMeasurement

[arnavk23]

Summary

Add a public accessor method pauli() to PauliProductMeasurement to expose the underlying Pauli object measured by the instruction.

Details and comments

This PR implements the feature requested in #15468 by adding a public pauli() method to the PauliProductMeasurement class.

Implementation:

• Added pauli() -> Pauli method that reconstructs the Pauli product from internal z/x/phase components
• Returns the full Pauli including global phase (+1 or -1)
• Leverages existing internal storage (_pauli_z, _pauli_x, _pauli_phase)
• Maintains full backward compatibility with QPY serialization and synthesis code

Testing:

• Added test_pauli_accessor() unit test in test/python/circuit/library/test_ppm.py
• Verifies that PauliProductMeasurement(Pauli("-XZ")).pauli() == Pauli("-XZ")
• All 31 existing PauliProductMeasurement tests pass

Documentation:

• Added release note via reno in releasenotes/notes/allow-pauli-accessor-15468.yaml
• Public method includes comprehensive docstring for API documentation

Verification:

• Tested locally: 31/31 PauliProductMeasurement tests pass
• No changes to serialization, synthesis, or other code paths

Fixes #15468

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[coveralls]

Pull Request Test Coverage Report for Build 20375939943

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 12 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.01%) to 88.323%

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Files with Coverage Reduction	New Missed Lines	%
crates/circuit/src/parameter/parameter_expression.rs	1	82.3%
crates/qasm2/src/lex.rs	1	93.32%
crates/circuit/src/parameter/symbol_expr.rs	10	72.9%

Totals
Change from base Build 20366546417:	0.01%
Covered Lines:	96731
Relevant Lines:	109519

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💛 - Coveralls