fix: align Go-plugin :report JSON keys with bash-strip convention

Every Go-implemented plugin's `:report --format json` now emits keys without the redundant `<plugin>-` head segment, matching the shape bash plugins have always emitted. The CLI flag names and `:set` semantics are unchanged. For backwards compatibility during the 0.38.x patch series, the legacy `<plugin>-<property>` keys are emitted side-by-side with the new keys, and a future major release will drop the legacy keys. `common.ReportSingleApp` is refactored to accept a `ReportSingleAppInput` struct with a `Validate()` method so the input is checked before any work runs, which also catches the latent `"docker options"` reportType bug at the API boundary.

tests/unit/storage.bats

@@ -486,3 +486,17 @@ teardown() {
   run /bin/bash -c "sudo test -e $flag_dir/$TEST_APP"
   assert_failure
 }
+
+@test "(storage:report) emits new stripped JSON keys alongside legacy" {
+  run /bin/bash -c "dokku storage:report $TEST_APP --format json | jq -r 'has(\"build-mounts\") and has(\"storage-build-mounts\")'"
+  assert_success
+  assert_output "true"
+
+  run /bin/bash -c "dokku storage:report $TEST_APP --format json | jq -r 'has(\"deploy-mounts\") and has(\"storage-deploy-mounts\")'"
+  assert_success
+  assert_output "true"
+
+  run /bin/bash -c "dokku storage:report $TEST_APP --format json | jq -r 'has(\"run-mounts\") and has(\"storage-run-mounts\")'"
+  assert_success
+  assert_output "true"
+}