Use LinearSolve's internal methods for preconditioners

[albertomercurio]

Checklist

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Description

The previous implementation was handling the left and right preconditioners externally. However, this can be directly done in the solver definition, simplifying the syntax.

[Copilot]

Pull Request Overview

This PR refactors the SteadyStateLinearSolver to use LinearSolve's internal preconditioner methods instead of handling preconditioners externally. The Pl and Pr parameters are removed from the solver struct, and preconditioners are now defined directly within the algorithm specification.

Key changes:

• Removed Pl and Pr fields from SteadyStateLinearSolver struct
• Changed default algorithm from KrylovJL_GMRES() to KrylovJL_GMRES(; precs = (A, p) -> (ilu(A, τ = 0.01), I))
• Removed external preconditioner application logic from _steadystate and _steadystate_fourier functions
• Updated LinearProblem constructor calls to use LinearProblem{true} for optimization

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/steadystate.jl	Simplified SteadyStateLinearSolver struct by removing preconditioner fields; updated default algorithm to include built-in preconditioner; removed manual preconditioner application logic
docs/src/users_guide/steadystate.md	Updated example code to reflect removal of Pl and Pr parameters
CHANGELOG.md	Added entry documenting the change to use LinearSolve's internal preconditioner methods

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[ytdHuang]

@TendonFFF

Can you also apply the same thing for HEOM ?