test(backend): add process manager and steamcmd tests with stub scripts

- server_process_test.go: 16 tests covering splitArgs, hasArgPrefix,
  buildAutoArgs (config/port/profiles), buildArgs, resolveModPath (name/
  ID/empty), buildModPath, WriteUserconfigFiles, Start/Stop lifecycle,
  percent-command substitution
- steamcmd_test.go: 9 tests covering CheckWorkshopMod (directory/file/
  empty), SteamCmdManager state machine, DownloadMods/UpdateGame
- testdata/: stub arma3server_x64 and steamcmd scripts for safe process
  testing without real binaries

backend/internal/services/testdata/arma3server_x64

@@ -0,0 +1,5 @@
+#!/bin/sh
+# Stub arma3server binary for testing.
+# Logs args to stdout then sleeps until killed (simulates a running server).
+echo "[STUB] arma3server_x64 called with: $@"
+sleep 3600

backend/internal/services/testdata/steamcmd

@@ -0,0 +1,5 @@
+#!/bin/sh
+# Stub steamcmd binary for testing.
+# Logs args to stdout then exits immediately.
+echo "[STUB] steamcmd called with: $@"
+exit 0