🐛 integrate: several OdeSolver fixes

[jorenham]

• add missing y attribute
• fix incorrect constructor parameter annotation for t0
• complex128 support
• .dense_output() scalar-type specialization
• slightly incorrect .direction and .step_size types

Closes #879

[Copilot]

Pull Request Overview

This PR fixes several issues in the ODE solver type stubs to improve type accuracy and consistency. The changes focus on proper generic parameterization, adding missing attributes, and correcting type annotations.

• Add missing y attribute to OdeSolver class
• Parameterize DenseOutput classes with scalar types for better type safety
• Fix incorrect type annotations for constructor parameters and method return types

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
scipy-stubs/integrate/_ivp/base.pyi	Major refactoring of base classes with generic parameterization, added missing y attribute, and improved type annotations
scipy-stubs/integrate/_ivp/rk.pyi	Updated RkDenseOutput and Dop853DenseOutput to inherit from parameterized DenseOutput
scipy-stubs/integrate/_ivp/radau.pyi	Updated RadauDenseOutput to inherit from DenseOutput[np.float64]
scipy-stubs/integrate/_ivp/lsoda.pyi	Updated LsodaDenseOutput to inherit from DenseOutput[np.float64]
scipy-stubs/integrate/_ivp/bdf.pyi	Updated BdfDenseOutput to inherit from DenseOutput[np.float64]
scipy-stubs/integrate/_ivp/ivp.pyi	Updated function parameter to use parameterized DenseOutput[Any]
scipy-stubs/integrate/_ivp/common.pyi	Updated type variable bound to use parameterized DenseOutput

[github-actions]

mypy_primer results

No ecosystem changes detected ✅