Add version guards to Enzyme test files to prevent loading on Julia 1.12

Added defensive guards at the top of Enzyme test files to explicitly skip
tests on Julia 1.12+ prerelease versions. This prevents accidental Enzyme
loading if these test files are run directly outside of the normal test
harness.

While the CI workflow and test/runtests.jl already prevent Enzyme tests
from running on v1.12, these file-level guards provide an additional
safety layer and make the version requirement explicit in each test file.

Affected files:
- test/enzyme/autodiff_events.jl
- test/enzyme/discrete_adjoints.jl

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

test/enzyme/autodiff_events.jl

@@ -1,3 +1,9 @@
+# Skip Enzyme tests on Julia 1.12+ prerelease versions
+if !isempty(VERSION.prerelease)
+    @warn "Skipping Enzyme tests on Julia prerelease version $(VERSION)"
+    exit(0)
+end
+
 using SciMLSensitivity
 using OrdinaryDiffEq, OrdinaryDiffEqCore, FiniteDiff, Test
 using Zygote

test/enzyme/discrete_adjoints.jl

@@ -1,3 +1,9 @@
+# Skip Enzyme tests on Julia 1.12+ prerelease versions
+if !isempty(VERSION.prerelease)
+    @warn "Skipping Enzyme tests on Julia prerelease version $(VERSION)"
+    exit(0)
+end
+
 using Enzyme, OrdinaryDiffEqTsit5, StaticArrays, DiffEqBase, ForwardDiff, Test
 
 function lorenz!(du, u, p, t)