rag.ask_contexts() cannot succeed: backend returns UNIMPLEMENTED on v1 and a generic INVALID_ARGUMENT on v1beta1. Is AskContexts allowlisted?

### Summary

`agentplatform` (shipped in `google-cloud-aiplatform`) exposes
`client.rag.ask_contexts()`. Every call returns `400 INVALID_ARGUMENT` with no
field named.

This does not appear to be a client defect. I checked the request the SDK builds
against the discovery schema and it matches. The same request sent by hand fails
identically, and the `v1` form of the method answers:

```
501 UNIMPLEMENTED
"Operation is not implemented, or supported, or enabled."
```

So the question is whether `AskContexts` requires allowlisting, and if so whether
the SDK should surface the method (or raise something more useful) until it is
generally served.

### Environment

- `google-cloud-aiplatform` 1.165.1 (also reproduced on 1.163.0)
- `google-genai` 2.16.0
- Python 3.13
- Regions tested: us-west1, us-central1, us-east4, europe-west4

### What I measured

Live RAG corpus in us-west1, 4 documents, state ACTIVE. Same credentials and
same client for every row.

| probe | result |
|---|---|
| `retrieve_contexts`, same corpus, creds, region | OK, 4 chunks |
| `ask_contexts()` via the SDK | 400 INVALID_ARGUMENT, no field named |
| raw REST, `.../locations/L:askContexts` | 400, identical |
| body `{"query": {...}}` only (minimal valid per schema) | 400 |
| body `{}` | 400 |
| body naming a corpus that does not exist | 400, identical to the valid one |
| body with an unknown field | 400 `Unknown name "zzz": Cannot find field` |
| us-central1, us-east4, europe-west4 | 400 in all three |
| same method on `v1` | **501 UNIMPLEMENTED** |
| control: `retrieveContexts` with `{}` | 400 `"request must set datasource."` |

Two rows are the interesting ones:

1. The endpoint **parses** the body, because it names an unknown field precisely.
2. It **never reaches corpus validation**, because a nonexistent corpus and a
valid corpus produce byte-identical errors.

And the control shows the RAG service returns specific, actionable errors in this
region when it is actually serving a method.

### The request the SDK builds is schema-correct

`GoogleCloudAiplatformV1beta1AskContextsRequest` declares exactly `{query, tools}`.
That is what the SDK sends, and the corpus does reach the server inside `tools`:

```python
from agentplatform._genai import rag, types as T
from google.genai import types as G

CORPUS = "projects/PROJECT/locations/us-west1/ragCorpora/CORPUS_ID"
tools = [G.Tool(retrieval=G.Retrieval(vertex_rag_store=G.VertexRagStore(
rag_resources=[G.VertexRagStoreRagResource(rag_corpus=CORPUS)])))]

params = T._AskContextsRequestParameters(
query=T.RagQuery(text="example question"),
config=T.AskContextsConfig(tools=tools))

d = rag._AskContextsRequestParameters_to_vertex(params)
d.pop("config", None) # the call site does this
print(sorted(d)) # ['query', 'tools']
print(CORPUS in str(d)) # True
```

### Reproduction

```python
import agentplatform
from agentplatform._genai import types as T
from google.genai import types as G

c = agentplatform.Client(project="PROJECT", location="us-west1")
CORPUS = "projects/PROJECT/locations/us-west1/ragCorpora/CORPUS_ID"

# works
c.rag.retrieve_contexts(
vertex_rag_store=G.VertexRagStore(
rag_resources=[G.VertexRagStoreRagResource(rag_corpus=CORPUS)]),
query=T.RagQuery(text="example question"))

# 400 INVALID_ARGUMENT, no field named
c.rag.ask_contexts(
query=T.RagQuery(text="example question"),
config=T.AskContextsConfig(tools=[G.Tool(retrieval=G.Retrieval(
vertex_rag_store=G.VertexRagStore(
rag_resources=[G.VertexRagStoreRagResource(rag_corpus=CORPUS)])))]))
```

And the v1 form, which is the clearest signal:

```
POST https://us-west1-aiplatform.googleapis.com/v1/projects/PROJECT/locations/us-west1:askContexts
{"query": {"text": "example question"}}

501 {"error": {"code": 501,
"message": "Operation is not implemented, or supported, or enabled.",
"status": "UNIMPLEMENTED"}}
```

### Questions

1. Is `AskContexts` allowlisted or otherwise gated? If so, what is the path to
enablement?
2. If it is not generally available, could the v1beta1 endpoint return the same
`UNIMPLEMENTED` that v1 returns, instead of a generic `INVALID_ARGUMENT` that
sends callers looking for a bad field?

### Note on a non-issue

While investigating I noticed the built path is
`projects/P/locations/L/:askContexts`, with a separator before the colon. That is
**not** the cause: `retrieve_contexts` builds `projects/P/locations/L/:retrieveContexts`
in exactly the same way and works. Mentioning it only so nobody else spends time
on it.

Start by reading the agentplatform._genai.rag AskContexts request conversion and comparing it with the retrieve_contexts path; reproduce the v1beta1 and v1 responses described in the issue. Done means the service's allowlisting status and expected SDK behavior are established, with any proposed change backed by a focused test or documented response.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.