Narrow range expansion to !isadaptive(alg) and non-CompositeAlgorithm

Two more CI regressions surfaced:

1. `composite_algorithm_test.jl` \"Mixed adaptivity\": the test asserts
   bit-exact `sol3.u == sol4.u` between two CompositeAlgorithms that
   differ only in sub-alg ordering plus a swapped choice function. With
   `dt=0.05, adaptive=false`, accumulated drift previously kept
   `t < 0.5` throughout the pre-switch phase; range tstops instead land
   exactly on `t = 0.5`, which the two choice functions discriminate
   differently (`t < 0.5` vs `t > 0.5`), routing the two composites
   through different sub-algorithms at the boundary. Skip composite
   algorithms.

2. `OrdinaryDiffEqLowStorageRK` RDPK3SpFSAL510 convergence: for
   adaptive algorithms run with `adaptive=false` (what
   `DiffEqDevTools.test_convergence` does), landing exactly on every
   range tstop changes the per-step error enough to push the fitted
   convergence order from 5.05 (master) to 5.30 — outside the ±0.25
   tolerance. Restricting the expansion to `!isadaptive(alg)` leaves
   adaptive-with-adaptive=false solvers on the prior code path. The
   PositiveIntegrators.jl bug case (`Euler`) still fires because
   `isadaptive(Euler()) == false`.

Verified locally: Euler bug produces 11 steps, composite test passes,
RDPK3 order back to 5.048, `tspan=(0,Inf)` + `set_proposed_dt!` cases
still work.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>

Author: ChrisRackauckas

lib/OrdinaryDiffEqCore/src/solve.jl

@@ -411,8 +411,9 @@ function _ode_init(
     # d_discontinuities, or callbacks (which may mutate dt at runtime via
     # `set_proposed_dt!`) to preserve existing "step at dtcache" semantics,
     # and when tspan has non-finite endpoints.
-    if !isnothing(dt) && !iszero(dt) && (!isadaptive(_alg) || !adaptive) &&
-            isdtchangeable(_alg) && sign(dt) == tdir &&
+    if !isnothing(dt) && !iszero(dt) && !isadaptive(_alg) &&
+            isdtchangeable(_alg) && !(_alg isa CompositeAlgorithm) &&
+            sign(dt) == tdir &&
             isempty(tstops) && isempty(d_discontinuities) &&
             callback === nothing && all(isfinite, tspan)
         tstops = ((tspan[1] + dt):dt:tspan[end]...,)