AppProxyClient endpoint methods do not surface error responses as domain exceptions

## Summary

`AppProxyClient` in `src/ai/backend/manager/clients/appproxy/client.py` has inconsistent error handling across its methods. Only `fetch_status` maps low-level `aiohttp` exceptions to the `AppProxyConnectionError` / `AppProxyResponseError` domain exceptions defined in `manager/errors/appproxy.py`. The four endpoint methods — `create_endpoint`, `create_endpoints_bulk`, `delete_endpoint`, `delete_endpoints_bulk` — leak raw aiohttp exceptions or, in the case of `delete_endpoint`, silently swallow non-2xx responses entirely.

This was flagged during review of #11328 (BA-1929) as an out-of-scope but real robustness gap.

## Concrete defects

1. **`delete_endpoint`** uses `async with ... as resp: pass` with no `resp.raise_for_status()` and no body read. A 4xx/5xx response from the coordinator is silently dropped — the manager logs and returns successfully, even though the deletion never happened.
2. **`create_endpoint`, `create_endpoints_bulk`, `delete_endpoints_bulk`** call `resp.raise_for_status()` followed by `await resp.json()`. Both can raise raw `aiohttp.ClientResponseError` / `aiohttp.ContentTypeError`, neither of which is wrapped into a `BackendAIError` subclass. Callers in `manager/sokovan/deployment/executor.py` then see a non-domain exception, and the eventual `DeploymentExecutionError` does not carry the AppProxy domain.
3. The structured `BackendAIError` JSON body returned by the coordinator on validation failures (after #11329) is not preserved through to the caller. The error gets re-raised as an aiohttp exception with only the status code.

## Proposed fix

Apply the same `try / except (ClientConnectorError, ClientResponseError, ContentTypeError, JSONDecodeError)` pattern that `fetch_status` uses to all four endpoint methods, and add `raise_for_status()` to `delete_endpoint`. Where possible, attach the parsed coordinator error body to `AppProxyResponseError.extra_data` so the upstream JSON error survives the translation.

## Out of scope

- Re-architecting the resilience policy or retry behavior.
- Changing the public signatures of the four methods.

## Related

- #5228 — original bug report (HTML vs JSON)
- #11328 — manager-side `Accept` header fix (BA-1929)
- #11329 — coordinator-side default-to-JSON fix (BA-1929)