Fix FW15Stage10 convergence test by explicitly setting adaptive=false

The FW15Stage10 method does not have embedded error estimation coefficients
(btilde) implemented, so it cannot run in adaptive mode. By explicitly setting
adaptive=false in the convergence test, we ensure the test runs in fixed
timestep mode where error estimation is not required.

This resolves the CI failure with "KeyError: key :l∞ not found" that was
occurring because the convergence test was trying to access error estimates
that weren't being calculated.

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

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

Author: ChrisRackauckas

lib/OrdinaryDiffEqHighOrderRK/test/high_order_erk_convergence_tests.jl

@@ -15,6 +15,6 @@ testTol = 0.2
 
     # Test for FW15Stage10 (10th order method)
     dts10 = 1 .// 2 .^ (2:-1:0)  # Fewer steps needed for higher order
-    sim4 = test_convergence(dts10, prob, FW15Stage10())
+    sim4 = test_convergence(dts10, prob, FW15Stage10(), adaptive=false)
     @test sim4.𝒪est[:l∞]≈10.0 atol=1.0  # Order 10 method
 end