Add QASM3 export support for IonQ native gates

[dlyongemallo]

Summary

Adds QASM3 definitions for IonQ native gates so that output of qiskit.qasm3.dumps can be parsed by openqasm3.parser without error. Fixes #217 and #218.

Details and comments

Adds _define() methods to all four IonQ native gates (GPIGate, GPI2Gate, MSGate, ZZGate). This provides decompositions into standard Qiskit gates that QASM exporters can serialize.

Gate decompositions:

Gate	Decomposition
GPIGate(φ)	U(π, 2πφ, π - 2πφ)
GPI2Gate(φ)	U(π/2, 2πφ - π/2, π/2 - 2πφ)
MSGate(φ0, φ1, θ)	RZ + RXX + RZ sequence
ZZGate(θ)	RZZ(2πθ)

Testing:

• Added parametric tests verifying matrix equivalence between each gate and its decomposition.
• Added integration tests for QASM3 export and parsing with openqasm3.parser.
• Added test reproducing the exact scenario from issue QASM3ExporterError / QASM3ParserError from IonQ native gate program #217.

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[Copilot]

Pull request overview

This pull request adds QASM3 export support for IonQ native gates by implementing _define() methods that decompose each gate into standard Qiskit gates. This resolves issues where circuits using IonQ native gates could not be exported to QASM3 format or were producing invalid QASM3 that failed to parse.

Changes:

• Added _define() methods to all four IonQ native gates (GPIGate, GPI2Gate, MSGate, ZZGate) providing decompositions into standard Qiskit gates
• Added comprehensive tests verifying mathematical equivalence between gates and their decompositions
• Added integration tests for QASM3 export and parsing, including exact reproduction of the reported issue scenario
• Added openqasm3[parser] test dependency

Reviewed changes

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

File	Description
qiskit_ionq/ionq_gates.py	Implements _define() methods for all four IonQ native gates, providing decompositions into U, RZ, RXX, and RZZ gates
test/ionq_gates/test_gates.py	Adds parametric tests verifying matrix equivalence of decompositions and integration tests for QASM3 export/parsing
requirements-test.txt	Adds openqasm3[parser] as a test dependency

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