test: backfill raw/global/computed report triplet coverage across plugins

The bats coverage for plugin :report keys has been uneven since the raw/global/computed split landed in PR #8640. This commit adds a single combined triplet test per settable property across app-json, builder, builder-dockerfile, builder-herokuish, builder-lambda, builder-nixpacks, builder-pack, builder-railpack, buildpacks, cron (maintenance), git (keep-git-dir, rev-env-var, source-image), logs (vector-sink, vector-global-image, vector-global-networks), network (attach-post-create, attach-post-deploy, initial-network), proxy (per-app type), ps (stop-timeout-seconds), registry (image-repo, push-on-release, push-extra-tags), storage (build/deploy/run mounts), and adds a new tests/unit/scheduler.bats and tests/unit/nginx-vhosts_properties.bats. Brings ps `procfile-path` and `stop-timeout-seconds` global getters in line with the #8640 convention (raw global, default resolved in computed). Documents the read-only and internal properties that surface in `:report` (or are written by Dokku) but cannot be managed by `:set` in a new `## Internal properties` section appended to each affected plugin's docs page.

docs/networking/proxy-management.md

@@ -236,3 +236,11 @@ Proxy implementations may decide to omit some functionality here, or use plugin
 Individual proxy implementations _may_ trigger app rebuilds, depending on how proxy metadata is exposed for the proxy implementation.
 
 Finally, proxy implementations _may_ install extra software needed for the proxy itself in whatever manner deemed fit. Proxy software can run on the host itself or within a running Docker container with either exposed ports or host networking.
+
+## Internal properties
+
+The following properties surface in `proxy:report` but are not managed by `proxy:set`:
+
+| Property | Kind | Description | Source |
+|---|---|---|---|
+| `--proxy-enabled` | read-only | `true` when the app's `disabled` property is not `true` | `plugins/proxy/proxy.go::IsAppProxyEnabled` |