Migrate AD backends to a group env (test/Extensions/Project.toml) (#552)

Move the heavy optional AD/analysis backends Mooncake, Zygote, and
SparseConnectivityTracer out of the root package's old-style
[extras]/[targets].test and into a dedicated group env
test/Extensions/Project.toml (the group whose test files actually
`using` them). All three are used only in test/Extensions/*.jl.

This stops the root/Core Downgrade lane from floor-resolving the
Mooncake/Zygote joint dependency graph: the jdc-52 downgrade harness
drops these three from its "Independently downgrading N weakdep
extension(s)" set (7 -> 4) and no longer writes Manifest-Mooncake,
Manifest-Zygote, or Manifest-SparseConnectivityTracer, while the base
project still resolves. The cross-backend AD tests are unchanged; the
SciMLTesting folder harness auto-activates the group env, so
runtests.jl (run_tests()) needs no wiring.

All three moved packages remain [weakdeps] (they gate extensions), so
their root [compat] entries are kept (no dangling-compat resolver
error). No package is a pure test-only extra here, so no [compat]
deletion is needed.

Co-authored-by: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>

Author: ChrisRackauckas-Claude

Project.toml

@@ -67,17 +67,14 @@ ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
 FiniteDifferences = "26cc04aa-876d-5657-8c51-4c34ba976000"
 ForwardDiff = "f6369f11-7733-5829-9624-2563aa707210"
 Makie = "ee78f7c6-11fb-53f2-987a-cfe4a2b5a57a"
-Mooncake = "da2b9cff-9c12-43a0-ae48-6db2b0edb7d6"
 Optim = "429524aa-4258-5aef-a3af-852621145aeb"
 QuadGK = "1fd47b50-473d-5c70-9696-f719f8f3bcdc"
 SafeTestsets = "1bc83da4-3b8d-516f-aca4-4fe02f6d838f"
 SciMLTesting = "09d9d899-5365-40a9-917a-5f67fddea283"
-SparseConnectivityTracer = "9f842d2f-2579-4b1d-911e-f412cf18a3f5"
 StableRNGs = "860ef19b-820b-49d6-a774-d7a799459cd3"
 Symbolics = "0c5d862f-8b57-4792-8d23-62f2024744c7"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 Unitful = "1986cc42-f94f-5a68-af5c-568840ba703d"
-Zygote = "e88e6eb3-aa80-5325-afca-941959d7151f"
 
 [targets]
-test = ["Aqua", "AllocCheck", "BenchmarkTools", "SafeTestsets", "SciMLTesting", "ChainRulesCore", "Mooncake", "Optim", "Test", "StableRNGs", "FiniteDifferences", "QuadGK", "ForwardDiff", "Symbolics", "Unitful", "Zygote", "SparseConnectivityTracer"]
+test = ["Aqua", "AllocCheck", "BenchmarkTools", "SafeTestsets", "SciMLTesting", "ChainRulesCore", "Optim", "Test", "StableRNGs", "FiniteDifferences", "QuadGK", "ForwardDiff", "Symbolics", "Unitful"]

test/Extensions/Project.toml

@@ -0,0 +1,25 @@
+[deps]
+DataInterpolations = "82cc6244-b520-54b8-b5a6-8a565e85f1d0"
+ForwardDiff = "f6369f11-7733-5829-9624-2563aa707210"
+LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
+Mooncake = "da2b9cff-9c12-43a0-ae48-6db2b0edb7d6"
+SafeTestsets = "1bc83da4-3b8d-516f-aca4-4fe02f6d838f"
+SciMLTesting = "09d9d899-5365-40a9-917a-5f67fddea283"
+SparseConnectivityTracer = "9f842d2f-2579-4b1d-911e-f412cf18a3f5"
+Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
+Zygote = "e88e6eb3-aa80-5325-afca-941959d7151f"
+
+[sources]
+DataInterpolations = {path = "../.."}
+
+[compat]
+DataInterpolations = "9"
+ForwardDiff = "1"
+LinearAlgebra = "1.10"
+Mooncake = "0.4.175, 0.5"
+SafeTestsets = "0.1"
+SciMLTesting = "1.7"
+SparseConnectivityTracer = "1"
+Test = "1.10"
+Zygote = "0.6.77, 0.7"
+julia = "1.10"