Fix completion at the end of a complete line

[IanButterworth]

Fixes JuliaLang/julia#58690
Fixes JuliaLang/julia#58103

Using Claude.

[nsajko]

repl.jl: Error During Test at /Users/runner/work/Pkg.jl/Pkg.jl/test/runtests.jl:83
  Got exception outside of a @test
  LoadError: syntax: "using" expression not at top level
  Stacktrace:
    [1] top-level scope
      @ ~/work/Pkg.jl/Pkg.jl/test/repl.jl:840
    [2] include(mapexpr::Function, mod::Module, _path::String)
      @ Base ./Base.jl:309
    [3] IncludeInto
      @ ./Base.jl:310 [inlined]
    [4] macro expansion
      @ ~/work/Pkg.jl/Pkg.jl/test/runtests.jl:86 [inlined]
    [5] macro expansion
      @ ~/hostedtoolcache/julia/nightly/aarch64/share/julia/stdlib/v1.13/Test/src/Test.jl:2019 [inlined]
    [6] macro expansion
      @ ~/work/Pkg.jl/Pkg.jl/test/runtests.jl:83 [inlined]
    [7] macro expansion
      @ ~/hostedtoolcache/julia/nightly/aarch64/share/julia/stdlib/v1.13/Test/src/Test.jl:1930 [inlined]
    [8] (::Main.PkgTestsOuter.PkgTestsInner.var"#2#3")()
      @ Main.PkgTestsOuter.PkgTestsInner ~/work/Pkg.jl/Pkg.jl/test/runtests.jl:81
    [9] with_logstate(f::Main.PkgTestsOuter.PkgTestsInner.var"#2#3", logstate::Base.CoreLogging.LogState)
      @ Base.CoreLogging ./logging/logging.jl:540
   [10] with_logger(f::Function, logger::Base.CoreLogging.ConsoleLogger)
      @ Base.CoreLogging ./logging/logging.jl:651
   [11] top-level scope
      @ ~/work/Pkg.jl/Pkg.jl/test/runtests.jl:38
   [12] include(mapexpr::Function, mod::Module, _path::String)
      @ Base ./Base.jl:309
   [13] top-level scope
      @ none:6
   [14] eval(m::Module, e::Any)
      @ Core ./boot.jl:489
   [15] exec_options(opts::Base.JLOptions)
      @ Base ./client.jl:296
   [16] _start()
      @ Base ./client.jl:563
  in expression starting at /Users/runner/work/Pkg.jl/Pkg.jl/test/repl.jl:3

[IanButterworth]

I marked it as draft as it's not ready for review. Just wanted to get this opened before I was AFK.

I've not reviewed the test code yet.