stage #4593 onto master (v0.51.550) — document extension sidecar CSP support + CSP test coverage

santastabber: docs (README + EXTENSIONS.md) clarifying extension sidecar CSP behavior +
test_issue1909_csp_enforcement / test_issue2901_csp_connect_extra assertions. Docs+tests only, no
prod code. Co-authored-by: santastabber <santastabber@users.noreply.github.com>

README.md

@@ -341,7 +341,7 @@ Full list of environment variables:
 | `HERMES_WEBUI_DEFAULT_WORKSPACE` | `~/workspace` | Default workspace |
 | `HERMES_WEBUI_DEFAULT_MODEL` | *(provider default)* | Optional model override; leave unset to use the active Hermes provider default |
 | `HERMES_WEBUI_PASSWORD` | *(unset)* | Set to enable password authentication |
-| `HERMES_WEBUI_CSP_CONNECT_EXTRA` | *(unset)* | Optional space-separated `http(s)://` or `ws(s)://` origins to append to the report-only CSP `connect-src` directive for reverse-proxy or tunnel deployments |
+| `HERMES_WEBUI_CSP_CONNECT_EXTRA` | *(unset)* | Optional space-separated `http(s)://` or `ws(s)://` origins to append to the enforced and report-only CSP `connect-src` directives for trusted reverse-proxy, tunnel, or extension sidecar deployments |
 | `HERMES_WEBUI_SSE_CHUNKED` | *(unset)* | Set truthy (`1`/`true`/`yes`/`on`) to send SSE with `Transfer-Encoding: chunked`. Needed behind buffering reverse proxies (e.g. `jupyter-server-proxy`) that otherwise buffer the whole stream; harmless but unnecessary for directly-served deployments |
 | `HERMES_WEBUI_EXTENSION_DIR` | *(unset)* | Optional local directory served at `/extensions/`; must point to an existing directory before extension injection is enabled |
 | `HERMES_WEBUI_EXTENSION_MANIFEST` | *(unset)* | Optional relative JSON manifest inside `HERMES_WEBUI_EXTENSION_DIR` listing bundled scripts/styles to inject; see [WebUI Extensions](docs/EXTENSIONS.md) |