Add Pauli Y and Z gates

[blackboxprogramming]

Summary

• add Pauli-Y and Pauli-Z gate helpers to the quantum circuit simulator
• document the expanded gate set in the README
• extend the test suite to cover the new gates and their phase behaviour

Testing

• pytest -q

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[Copilot]

Pull request overview

This PR extends the quantum circuit simulator with two additional Pauli gates (Y and Z), expanding the available single-qubit operations beyond the existing Hadamard and Pauli-X gates.

• Added Pauli-Y and Pauli-Z gate methods to the QuantumCircuit class
• Implemented corresponding unitary matrices with correct complex phase factors
• Added comprehensive tests verifying both probability outcomes and phase behavior

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
quantum_simulator.py	Implements pauli_y() and pauli_z() methods with detailed docstrings and defines their matrix representations
tests/test_quantum_simulator.py	Adds two test cases verifying Y-gate bit-flip with phase and Z-gate phase modification without probability changes
README.md	Updates feature description to include Pauli-X/Y/Z gates in the gate set listing

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