### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### The Feature

Add native Gemini-API `cachedContents` CRUD routes to `litellm/proxy/google_endpoints/endpoints.py`, mirroring the pattern already shipped for `interactions` in the same file:

- `POST /v1beta/cachedContents` + `POST /cachedContents` - `GET /v1beta/cachedContents` + `GET /cachedContents` (list) - `GET /v1beta/cachedContents/{name:path}` + `GET /cachedContents/{name:path}` - `PATCH /v1beta/cachedContents/{name:path}` + `PATCH /cachedContents/{name:path}` (TTL extension) - `DELETE /v1beta/cachedContents/{name:path}` + `DELETE /cachedContents/{name:path}`

These should sign into the configured Vertex/Google credentials the same way `google_generate_content` already does, translate between Gemini-API resource names (`cachedContents/{id}`) and Vertex resource paths (`projects/{P}/locations/{L}/cachedContents/{id}`) as needed, and apply `user_api_key_auth` + `pre_call_hook` the same as the surrounding routes.

### Motivation, pitch

Today `google_endpoints/endpoints.py` registers `generateContent`, `streamGenerateContent`, `countTokens…