fix(gateway): grant sandboxclaims patch and map API errors to 503 (#45)

* fix(gateway): grant patch on sandboxclaims for token rotation

The SSO landing path calls K8sClient.patch_token(), which patches the
`dsh-<user>` SandboxClaim's token-hash annotation whenever the browser has
no valid gateway session: first visit, an expired or foreign cookie, or a
gateway restart that drops the in-memory session store. The gateway Role
only granted get/list/create/delete on sandboxclaims, so the patch returned
403 and, because ApiException is not handled, every such GET / answered a
bare 500 "Internal Server Error".

Grant the missing `patch` verb on
extensions.agents.x-k8s.io/sandboxclaims, and update the least-privilege
comment plus the two docs that enumerate the Role verbs.

Verified with `make check` (versions, example layout, manifests) and
`yamllint`; the 403 and the resulting 500 were reproduced from a running
install and cleared by applying the equivalent Role patch (RBAC is evaluated
per request, so no gateway restart is needed).

* fix(gateway): return 503 instead of a bare 500 on Kubernetes API errors

Every gateway route talks to the Kubernetes API, and ApiException used to be
unhandled: Starlette's ServerErrorMiddleware turned it into a bare
500 "Internal Server Error" plus an ERROR traceback. That masked a missing
RBAC verb as a client-visible crash on GET / (the SSO landing path), and it
would equally hide an API outage or throttling.

Register an ApiException handler that logs one credential-free line
(status/reason) and answers 503 + Retry-After with a fixed body, so the
Kubernetes Status payload (which can name cluster objects) is never proxied.
Guard the first get_claim() in the WebSocket route the same way: WebSocket
scopes bypass Starlette's HTTP exception handlers, so it now closes with 1011
instead of escaping as a traceback.

Tests assert the 503 + Retry-After response, cover the SSO landing path that
produced the reported 500 (forbidden claim patch), and assert the WebSocket
close code. The failure-semantics tables document the new behaviour.

---------

Co-authored-by: SREDevOpsAgents <agents@sredevops.org>

examples/dsh-agents-as-a-service/gateway/README.md

@@ -175,9 +175,11 @@ oauth2-proxy's egress to the gateway, DNS and public HTTPS.
 | Sandbox not provisioned, wake timeout, handshake failure | `503` + `Retry-After: 10`. The handshake error body is a fixed message: it never contains the launch token, the handshake URL or pod/credential details. |
 | User over `MAX_IN_FLIGHT_PER_USER` | `503` + `Retry-After` (HTTP); the existing in-flight requests are untouched |
 | Upstream connection/protocol failure | `502` |
+| Kubernetes API error (API unreachable/throttled, RBAC `403`, `5xx`) | `503` + `Retry-After: 10`. The body is a fixed message; the Kubernetes `Status` payload (which can name cluster objects) is never proxied, and the API `status`/`reason` is logged once |
 | WS auth failure (unknown user, bad token, cross-origin) | accept, then close `4401` |
 | WS per-user concurrency cap | accept, then close `1011` |
 | WS upstream/handshake failure | accept, then close `1011` |
+| WS Kubernetes API error | accept, then close `1011` |
 | WS message from DSH over `WS_MAX_MESSAGE_BYTES` | the upstream/relay closes the connection (websockets' message-too-big path) |
 
 A close code can only be delivered after the WebSocket upgrade is accepted;
@@ -230,7 +232,8 @@ would never be sent). HTTPS deployments must set `COOKIE_SECURE=true`.
 
 The gateway's ServiceAccount needs, in its namespace:
 
-* `sandboxclaims` (`extensions.agents.x-k8s.io`): get, list, create, delete
+* `sandboxclaims` (`extensions.agents.x-k8s.io`): get, list, create, patch, delete
+  (patch rotates the token-hash annotation when a session is minted)
 * `sandboxes` (`agents.x-k8s.io`): get, list, patch
 * `pods`: get, list
 * `pods/log`: get