feat: support optimal control problems with zero-dimensional control

[ocots]

📖 Summary

This PR introduces support for non-controlled optimal control problems (zero-dimensional control) by removing the artificial requirement that control variables must be declared before dynamics and cost criteria. The change enables mathematically valid problems without control variables while maintaining logical declaration order consistency.

🎯 Key Changes

Parser Flexibility

• Remove mandatory control validation in p_dynamics!, p_dynamics_coord!, p_lagrange!, and p_bolza! functions
• Add reverse validation in p_control! to prevent declaring control after dynamics/cost are defined
• Maintain declaration order logic while allowing autonomous problems

Technical Improvements

• Enhanced replace_call function to handle Vector{Union{Nothing, Symbol}} efficiently
• Skip Nothing symbols in replacement loops for better performance
• Improved type stability for variable substitution in autonomous systems

Comprehensive Testing

• Complete test suite for zero control dimension scenarios
• Error handling validation for invalid control initialization attempts
• Integration tests for solution building and serialization without control
• Module isolation improvements in test runner configuration

🔄 Behavior Changes

✅ Now Supported

# Autonomous optimal control problems
CTParser.@def begin
    t ∈ [0, 1], time
    x ∈ R², state
    ẋ(t) = [x₂(t), -x₁(t)]  # No control required
    x₁(1)^2 → min
end

❌ Still Prevented

# Invalid declaration order
CTParser.@def begin
    t ∈ [0, 1], time
    x ∈ R², state
    ẋ(t) = [x₂(t), u(t)]     # Dynamics first
    u ∈ R, control            # Control after dynamics ❌
    x₁(1)^2 → min
end

🧪 Testing

• 27 test cases covering all zero-control scenarios
• Error validation for improper control usage
• Integration verification with solution building pipeline
• Backwards compatibility maintained for existing control problems

📈 Impact

This change expands CTParser.jl's capability to handle the full spectrum of optimal control problems, from fully controlled to completely autonomous systems, while preserving the logical structure that prevents confusing mixed declaration patterns.

[ocots]

@jbcaillau Please review and merge.

[github-actions]

Breakage test results
Date: 2026-03-21 14:01:02

Name	Latest	Stable
OptimalControl	compat: v0.8.10-beta	compat: v0.8.10-beta