Drop DiffEqProblemLibrary QA self sources

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

Summary

• Remove committed self [sources] path entries from the root and sublibrary test/qa/Project.toml files.
• Keep the root QA activation on SciMLTesting.activate_group_env.
• For sublibraries, explicitly Pkg.develop the local sublibrary package before activating the QA environment, avoiding committed path sources without adding SciMLTesting to the outer test env.

Verification

• rg -n "^\[sources\]|path\s*=" test/qa/Project.toml lib/*/test/qa/Project.toml returned no matches.
• Runic passed on the touched runtests.jl files.
• timeout 3600 env GROUP=QA /home/crackauc/.juliaup/bin/julia +release --project=. --startup-file=no -e 'using Pkg; Pkg.test(; coverage=false)'\n - passed: root ExplicitImports | 18/18.\n- Sublibrary QA passed with DIFFEQPROBLEMLIBRARY_TEST_GROUP=QA:\n - BVProblemLibrary: Aqua 17/17\n - DAEProblemLibrary: Aqua 17/17\n - DDEProblemLibrary: Aqua 17/17\n - JumpProblemLibrary: Aqua 16/16\n - NonlinearProblemLibrary: Allocation 3/3, Aqua 17/17\n - ODEProblemLibrary: Aqua 17/17\n - SDEProblemLibrary: Aqua 16/16\n