Add comprehensive slice notation tests

[ocots]

• Test slices in constraints: x[1:3](0), x[1:end](0), x[1:n](0)
• Test slices in dynamics: ∂(x[1:3])(t), ∂(x[1:end])(t), ∂(x[1:n])(t)
• Test slices in Mayer cost: x[1:3](tf), x[1:end](tf), x[1:n](tf)
• Mark broken tests for unsupported features (end keyword, variable n)
• Literal integer ranges work for parsing, some broken for discretisation

[ocots]

@jbcaillau

Comprehensive Slice Notation Tests

This PR adds extensive tests for slice notation (x[...]) in different contexts of optimal control problem definitions.

Test Coverage

The test suite covers 3 main contexts where slices can appear:

1. Constraints (initial conditions): x[...](0) == x0
2. Dynamics: ∂(x[...])(t) == ...
3. Mayer cost (terminal cost): x[...](tf)

For each context, we test 3 slice variants:

• Literal integer range: x[1:3] ✅ Most reliable
• end keyword: x[1:end] ❌ Parsing fails
• Variable: x[1:n] where n=3 ⚠️ Context-dependent

Current Status

✅ What Works (End-to-End)

Context	Slice Type	Status
Constraints	x[1:3](0)	✅ Parsing + Discretisation + Solve

⚠️ What Partially Works (Parsing OK, Discretisation Fails)

Context	Slice Type	Parsing	Discretisation
Dynamics	∂(x[1:3])(t)	✅	❌
Dynamics	∂(x[1:n])(t)	✅	❌
Mayer cost	x[1:3](tf)	✅	❌
Mayer cost	x[1:end](tf)	✅	❌
Mayer cost	x[1:n](tf)	✅	❌

❌ What's Broken (Parsing Fails)

Context	Slice Type	Error
Constraints	x[1:end](0)	MethodError: no method matching (::Colon)(::Int64, ::Symbol)
Constraints	x[1:n](0)	MethodError: no method matching (::Colon)(::Int64, ::Symbol)
Dynamics	∂(x[1:end])(t)	MethodError: no method matching (::Colon)(::Int64, ::Symbol)

Key Findings

1. Literal integer ranges (x[1:3]) work everywhere for parsing
2. end keyword fails during parsing in constraints and dynamics contexts
3. Variable slices (x[1:n]) work for parsing in dynamics and Mayer cost, but fail in constraints
4. Discretisation currently only works for constraints with literal ranges

Test Structure

Each context has:

• Model builders: Helper functions to create test problems
• Unit tests: Verify parsing and model construction
• Integration tests: Test discretisation and solver execution

All broken tests are marked with @test_broken to document known limitations without failing the test suite.

Next Steps

1. High Priority: Fix end keyword support in slice parsing (affects all contexts)
2. High Priority: Fix variable slice support (x[1:n]) in constraints
3. Medium Priority: Fix discretisation for sliced dynamics (∂(x[1:3])(t))
4. Medium Priority: Fix discretisation for sliced Mayer costs (x[...](tf))

[github-actions]

Breakage test results
Date: 2026-04-10 14:13:39

Name	Latest	Stable
OptimalControl	compat: v0.8.13	compat: v0.8.13