Reformat generic_imex_perform_step.jl for Runic 1.9

[singhharsh1708]

The Runic / Runic Format Check job is failing on every open PR, including ones
that touch neither this file nor OrdinaryDiffEqSDIRK. Confirmed on #4356, #4357
and #4358, and on the older #4351 and #4354.

The single file it rejects is
lib/OrdinaryDiffEqSDIRK/src/generic_imex_perform_step.jl, at two continuation
lines inside an elseif condition:

elseif tab.stage1_extrapolation && (
        predictor == Predictor.Linear ||
-       (E === :ie_dd2 && !hasproperty(alg, :predictor))
+           (E === :ie_dd2 && !hasproperty(alg, :predictor))
    )

Runic 1.5 accepts the current indentation and Runic 1.9 does not, so this appeared
without the file being touched -- the CI action tracks the latest release.

This is the output of Runic.main(["--inplace", ...]) under 1.9.0, nothing hand
written. Formatting only, no behaviour change, so no version bump.

With it applied, Runic.main(["--check", "."]) over the whole repository exits 0.

AI Disclosure

Claude assisted with this change.