fix(parser): reject constraint bounds that depend on v/x/u/t (#343)

[ocots]

Closes #343.

Stacked on #326 — this PR targets chore/deps-examodels-0.12 so it can be reviewed and adjusted independently.

Problem

A boundary constraint whose bound references the optimization variable, e.g. x₂(0) == v, was correctly rejected but with an internal gensym leaking into the message:

Line 7: x₂(0) == v
UndefVarError: `v##286` not defined in `Main`

Root cause: in parse! aliases are substituted first, so v becomes v##NNNN[1]. x₂(0) == v calls p_constraint!(p, p_ocp, e2, e1, e2) — the bounds e1/e3 are v##NNNN[1] — and p_constraint_fun! emits constraint!(ocp, :boundary; f=fun, lb=(v##NNNN[1]), ub=(v##NNNN[1]), …). lb/ub are evaluated once at build time, but the gensym is only a function-argument name (for the :fun backend it is never bound at all), hence the UndefVarError. p_constraint! never validated the bounds.

This matches the documented rule "constraint bounds must be effective, that is must not depend on a variable".

Change

p_constraint! now checks both bounds and, if either depends on v, the state, the control or the time, returns a CTBase.ParsingError naming the real cause:

the lower bound of a constraint must not depend on the variable; write a functional
constraint instead by moving the term to the constrained side (e.g. `x₂(0) - v == 0`
rather than `x₂(0) == v`)

The check is backend-agnostic (before the :fun / :exa dispatch), so v == x₂(0) and x₁(0) ≤ x₂(0) are caught too. The constrained side may still reference the variable (e.g. 0 ≤ r(0) - z ≤ 1) — only the bounds are restricted.

Public API

No change. An input that used to throw UndefVarError now throws CTBase.ParsingError; nothing that used to build changes.

Tests

• test/test_onepass_fun.jl: two @test_throws ParsingError cases plus a positive check that the documented work-around x₂(0) - v == 0 still builds.
• test/test_onepass_exa.jl: one matching @test_throws ParsingError for the :exa path.
• Full suite green locally: 2599 pass, 9 broken (pre-existing), 0 fail/error.

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