Test symbolic jump affects through public API

[ChrisRackauckas-Claude]

Summary

• replace a Catalyst test dependency on non-public ModelingToolkitBase AffectSystem internals with public mtkcompile and equations calls
• remove the now-unused Symbolics unwrap import
• keep the assertion that non-species stoichiometry produces a Pre-wrapped explicit affect

Root cause

ModelingToolkit commit 136462c52d4c983e4ba832893928f270bc399b47 changed the internal AffectSystem constructor to require parent_sys. Catalyst called that non-public constructor directly in symbolic_stoichiometry.jl, so both ModelingToolkit Catalyst Modeling lanes failed with UndefKeywordError.

A behavioral boundary check passed on parent 0b01708ffb4c545b888f859bdcbc505350f723ef, and git bisect identifies 136462c52d4c983e4ba832893928f270bc399b47 as the first bad commit.

Validation

• exact Julia 1.12.6 clean-base GROUP=Modeling reproduction: Symbolic Stoichiometry 20 passed, 1 errored before the fix
• focused parent-boundary block: passed on 0b01708ffb
• focused public-API regression block after the fix: passed
• full test/reactionsystem_core/symbolic_stoichiometry.jl after the fix: exited 0, including its 10,000-trajectory simulation tail
• Runic 1.7.0 check on touched ranges 1:6 and 210:239: passed
• git diff --check: passed

The repository-wide Runic 1.7.0 check still exits 1 on pre-existing formatting in this test file outside the touched ranges; this PR does not mix that broad formatting cleanup into the regression fix.

Please ignore this PR until it has been reviewed by @ChrisRackauckas.

[ChrisRackauckas-Claude]

CI classification after the required clean-master audit:

• The PR-relevant Julia 1 Modeling job passes, as do Simulation, Hybrid, Misc, Spatial, all prerelease groups, all extension groups, and the two-hour build.
• Julia 1 QA was terminated by runner shutdown/SIGTERM. The exact clean Catalyst master QA command passed locally: Quality Assurance 10 pass / 7 broken / 17 total, followed by JET 1455/1455.
• Julia LTS Modeling reproduces unchanged on clean Catalyst master: DSL Options 308 pass / 9 fail / 1 broken. A same-machine Julia 1.10 A/B changed only SymbolicUtils 4.37.1 to 4.37.2; parameter discovery changed from [k, k_obs, k_ratio, eps_val] with a passing assertion to [k] with the assertion false. The existing repairs are Catalyst Fix symbolic option parameter discovery #1503 and ModelingToolkit #4714; independent evidence is posted on both.

Neither red check is caused by this focused parent_sys/public-API change. No test was skipped, silenced, or loosened.