An unknown discretization scheme is thrown as a bare String, not a CTException

[ocots]

Summary

src/onepass.jl throws a String for an unknown scheme, so typeof(e) === String. Callers
cannot dispatch on it, and any code following the ecosystem's exception conventions has nothing
typed to catch.

Reproduction

using OptimalControl, ExaModels, NLPModelsIpopt

ocp = @def begin
    t ∈ [0, 1], time
    x = (q, v) ∈ R², state
    u ∈ R, control
    x(0) == [-1, 0]
    x(1) == [0, 0]
    ∂(q)(t) == v(t)
    ∂(v)(t) == u(t)
    ∫(0.5u(t)^2) → min
end

try
    solve(ocp, :exa; scheme=:gauss_legendre_2, display=false)
catch e
    println("typeof = ", typeof(e))
    println(e)
end

typeof = String
unknown numerical scheme: gauss_legendre_2 (possible choices are :euler, :euler_implicit, :midpoint, :trapeze)

Cause

src/onepass.jl:941, and identically at :1035 and :1106:

throw(
    "unknown numerical scheme: $scheme (possible choices are :euler, :euler_implicit, :midpoint, :trapeze)",
) # (vs. __throw) since raised at runtime (and __wrap-ped)

The adjacent comment says the choice is deliberate — __throw is avoided because the error is
raised at runtime inside __wrap-ped generated code. But __wrap rethrows whatever it caught,
so the type of the thrown object is preserved either way; nothing about the runtime context
requires it to be untyped.

Why it matters

The ecosystem's stated rule is seven typed exceptions under CTException, chosen so callers can
catch precisely. Three consequences here:

1. catch e ... e isa CTBase.Exceptions.IncorrectArgument never matches.
2. showerror(io, e) falls back to show, so the message renders with surrounding quotes
   instead of the formatted │ Reason / Context / Hint block every other error in the ecosystem
   produces.
3. Concretely, this is the one exception demonstration in the OptimalControl documentation that
   cannot use the Handbook's prescribed @repl + showerror form — the other twenty-nine
   converted cleanly. See docs: display exceptions the way the Handbook prescribes OptimalControl.jl#878.

Suggested fix

CTBase.Exceptions.IncorrectArgument fits exactly: a single argument has an invalid value, and
the type carries got / expected / suggestion fields that already match what the message
spells out by hand.

throw(IncorrectArgument(
    "unknown numerical scheme";
    got = string(scheme),
    expected = ":euler, :euler_implicit, :midpoint or :trapeze",
))

🤖 Generated with Claude Code

[ocots]

This issue is addressed by PR #326: #326

The PR replaces the bare String exceptions with typed CTBase.IncorrectArgument / CTBase.PreconditionError exceptions. It covers the unknown numerical scheme errors as well as the other remaining bare-string error sites, and updates the corresponding tests.

The fix is implemented and the test suite is passing. This issue will be closed automatically once PR #326 is merged.