Add SymbolicUtils v4 compatibility

[ChrisRackauckas-Claude]

Summary

This PR adds compatibility with SymbolicUtils v4.

SymbolicUtils v4 renamed the Symbolic type to BasicSymbolic. This change:

1. Updates compat bounds to include SymbolicUtils v4
2. Updates the extension to use a version-agnostic SymbolicType constant that resolves to Symbolic (v0.19-v3) or BasicSymbolic (v4+)

Motivation

This is needed for DataDrivenSR (from DataDrivenDiffEq.jl) to work with SymbolicRegression.jl when using Symbolics v7 / SymbolicUtils v4.

Related: SciML/DataDrivenDiffEq.jl#571

Changes

• Project.toml: Added 4 to SymbolicUtils compat
• ext/SymbolicRegressionSymbolicUtilsExt.jl: Use conditional SymbolicType constant instead of directly importing Symbolic

Test plan

• Verify existing tests pass with SymbolicUtils v3
• Verify extension works with SymbolicUtils v4

🤖 Generated with Claude Code

[ChrisRackauckas]

This looks reasonable at face value.

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	master	241f615...	master / 241f615...
search/multithreading	14.8 ± 0.23 s	14.6 ± 0.15 s	1.01 ± 0.019
search/serial	32.9 ± 1.3 s	32.6 ± 0.81 s	1.01 ± 0.046
utils/best_of_sample	2.01 ± 0.6 μs	1.94 ± 0.52 μs	1.04 ± 0.42
utils/check_constraints_x10	16.6 ± 4.2 μs	16.9 ± 4.2 μs	0.982 ± 0.35
utils/compute_complexity_x10/Float64	2.16 ± 0.098 μs	2.15 ± 0.09 μs	1 ± 0.062
utils/compute_complexity_x10/Int64	2.1 ± 0.089 μs	2.06 ± 0.081 μs	1.02 ± 0.059
utils/compute_complexity_x10/nothing	1.55 ± 0.09 μs	1.55 ± 0.09 μs	1 ± 0.082
utils/insert_random_op_x10	5.39 ± 1.7 μs	5.33 ± 1.7 μs	1.01 ± 0.44
utils/next_generation_x100	0.437 ± 0.022 ms	0.44 ± 0.022 ms	0.993 ± 0.071
utils/optimize_constants_x10	0.0339 ± 0.0078 s	0.0332 ± 0.0075 s	1.02 ± 0.33
utils/randomly_rotate_tree_x10	8.17 ± 0.96 μs	7.89 ± 0.88 μs	1.03 ± 0.17
time_to_load	2.71 ± 0.14 s	2.62 ± 0.041 s	1.03 ± 0.055

Memory benchmarks

	master	241f615...	master / 241f615...
search/multithreading	0.207 G allocs: 54 GB	0.204 G allocs: 53.2 GB	1.02
search/serial	0.207 G allocs: 53.8 GB	0.207 G allocs: 53.8 GB	1
utils/best_of_sample	0.038 k allocs: 3.25 kB	0.038 k allocs: 3.25 kB	1
utils/check_constraints_x10	0.034 k allocs: 0.875 kB	0.034 k allocs: 0.875 kB	1
utils/compute_complexity_x10/Float64	0 allocs: 0 B	0 allocs: 0 B
utils/compute_complexity_x10/Int64	0 allocs: 0 B	0 allocs: 0 B
utils/compute_complexity_x10/nothing	0 allocs: 0 B	0 allocs: 0 B
utils/insert_random_op_x10	0.046 k allocs: 1.94 kB	0.045 k allocs: 1.88 kB	1.03
utils/next_generation_x100	4.63 k allocs: 0.276 MB	4.63 k allocs: 0.276 MB	1
utils/optimize_constants_x10	24.5 k allocs: 25.4 MB	21.5 k allocs: 21.3 MB	1.19
utils/randomly_rotate_tree_x10	0.042 k allocs: 1.34 kB	0.042 k allocs: 1.34 kB	1
time_to_load	0.145 k allocs: 11 kB	0.145 k allocs: 11 kB	1

[MilesCranmer]

Requires SymbolicML/DynamicExpressions.jl#148 first. (Not sure why those CI aren't running)