Fix FIRK step count test: use length(sol.t) instead of length(sol) (#3478)

On RAT v4, ODESolution is an AbstractArray. `length(sol)` dispatches to
`length(::AbstractArray)` which materializes the flattened array
dimensions. This JIT-compiles RecursiveArrayTools/FastBroadcast methods
for the solution's element type, changing how @.. broadcasts dispatch
for subsequent solves in the same process. The result: step counts
double from 117 to 234 for the Van der Pol μ≥1e7 cases.

`length(sol.t)` accesses the timestep vector directly without triggering
the AbstractArray machinery. This gives consistent results regardless of
compilation order.

Confirmed: Pkg.test passes 92/92 with this fix (previously 82/10).

Co-authored-by: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 3 people

lib/OrdinaryDiffEqFIRK/test/ode_firk_tests.jl

@@ -79,22 +79,22 @@ for iip in (true, false)
     @test length(sol.t) < 150
     @test SciMLBase.successful_retcode(sol)
     sol_temp = solve(remake(vanstiff, p = [1.0e7]), RadauIIA5())
-    @test length(sol_temp) < 150
+    @test length(sol_temp.t) < 150
     @test SciMLBase.successful_retcode(sol_temp)
     sol_temp2 = solve(remake(vanstiff, p = [1.0e7]), reltol = [1.0e-6, 1.0e-4], RadauIIA5())
-    @test length(sol_temp2) < 180
+    @test length(sol_temp2.t) < 180
     @test SciMLBase.successful_retcode(sol_temp2)
     sol_temp3 = solve(
         remake(vanstiff, p = [1.0e7]), RadauIIA5(), reltol = 1.0e-9,
         abstol = 1.0e-9
     )
-    @test length(sol_temp3) < 970
+    @test length(sol_temp3.t) < 970
     @test SciMLBase.successful_retcode(sol_temp3)
     sol_temp4 = solve(remake(vanstiff, p = [1.0e9]), RadauIIA5())
-    @test length(sol_temp4) < 170
+    @test length(sol_temp4.t) < 170
     @test SciMLBase.successful_retcode(sol_temp4)
     sol_temp5 = solve(remake(vanstiff, p = [1.0e10]), RadauIIA5())
-    @test length(sol_temp5) < 190
+    @test length(sol_temp5.t) < 190
     @test SciMLBase.successful_retcode(sol_temp5)
 end