Literate regeneration emits malformed Markdown for multiline #src test calls

origin/main (0ac4d92) contains multiline @test_reference calls whose final line carries #src, for example docs/src/literate_tutorials/diffusion_equation_on_an_annulus.jl:159-162.

Reproduction from a clean checkout:

LIVESERVER_ACTIVE=true julia +1.12 --project=docs -e 'using Pkg; Pkg.instantiate(); include("docs/make.jl")'

With the currently resolved Literate 2.21.0, regeneration writes incomplete @example and “Just the code” blocks. In docs/src/tutorials/diffusion_equation_on_an_annulus.md, the generated block retains:

@test_reference joinpath(
    @__DIR__, "../figures",
    "diffusion_equation_on_an_annulus_interpolated_with_naturalneighbours_bad.png"

The closing ) fig #src line is removed, making the generated Markdown invalid Julia. The pre-existing committed Markdown does not have this malformed fragment, so this is separate from the strict Documenter failures caused by stale solver imports.

Expected behavior: #src should suppress the full source-only test expression without emitting an incomplete partial expression, or the Literate sources should use a form that is stable under the current Literate release.

Start with the multiline @test_reference calls in docs/src/literate_tutorials/diffusion_equation_on_an_annulus.jl, especially lines 159-162, and reproduce with the provided Julia docs/make.jl command. Compare the generated blocks in docs/src/tutorials/diffusion_equation_on_an_annulus.md with the committed Markdown. Done means regeneration no longer emits incomplete Julia from #src-marked calls, either through a stable source form or the expected Literate behavior.