feat: Add RQang operation to parametric qubit operations

[LestlinRobins]

Closes #10171

Context

Single-qubit rotations in the XY plane are commonly used in quantum circuits and pulse-level control. While these rotations can already be constructed using combinations of existing gates, having a native operation makes them easier to work with, particularly when building and optimizing parametrized quantum circuits.

Description of the Change

This PR introduces RQang, a new parametric single-qubit operation in PennyLane for arbitrary rotations in the XY plane, parameterized by a rotation angle (\theta) and an azimuthal angle (\phi).

The implementation includes:

• Matrix representation and eigenvalue computation.
• Generator computation: G = 1/2 * (cos(phi) * X + sin(phi) * Y).
• Adjoint and power rules.
• Canonical decomposition: RZ(-phi) @ RX(theta) @ RZ(phi).
• Parameter validation using assert_valid.

The operation is exported through PennyLane's public API, with the necessary updates to pennylane/__init__.py, pennylane/ops/qubit/__init__.py, and tach.toml.

Unit tests have also been added to tests/ops/qubit/test_parametric_ops.py, along with a changelog entry in doc/releases/changelog-dev.md.

Benefits

• Provides a native operation for arbitrary XY-plane single-qubit rotations.
• Simplifies circuit construction by avoiding the need to manually combine multiple elementary gates.
• Supports differentiable parametrized quantum circuits and integrates with PennyLane's existing operator framework.

Possible Drawbacks

No specific drawbacks have been identified.

Related GitHub Issues

• Native qang unit/gate: qg_Z-parameterized rotation + calibration utilities #10171