Make java and neotest config portable

jdtls hardcoded /Library/Java/JavaVirtualMachines paths, which resolve to
nothing off macOS; discover JDKs across the macOS, Linux and sdkman roots
instead and only declare runtimes that exist. neotest-mocha passed `docker exec
-it`, which hangs without a TTY, and hardcoded the container name.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lua/plugins/neotest.lua

@@ -7,7 +7,7 @@ return {
     return {
       adapters = {
         ["neotest-mocha"] = {
-          command = "docker exec -it api-sandbox npm run test:integration",
+          command = "docker exec " .. (vim.env.NEOTEST_MOCHA_CONTAINER or "api-sandbox") .. " npm run test:integration",
           command_args = function(context)
             -- The context contains:
             --   results_path: The file that json results are written to