fix(discoveryengine): change streamAnswer return type to array

[AmariahAK]

Description

The streamAnswer REST endpoint returns a JSON array of
AnswerQueryResponse frames at runtime, but the generated TypeScript types
declared a single GoogleCloudDiscoveryengineV1AnswerQueryResponse object.
This caused res.data.answerQueryToken, res.data.answer?.relatedQuestions,
and similar fields to be undefined at runtime while TypeScript reported
them as present — because res.data is actually an array, not an object.

Root cause

The discovery document's response.$ref points to a single
AnswerQueryResponse, but the actual REST HTTP response body is a JSON
array. The generator faithfully produces the single-object type from the
discovery doc, which doesn't match reality. The discovery doc description
even says "returns multiple AnswerQueryResponse messages in a stream" —
the $ref just wasn't declared as an array.

Changes

• src/apis/discoveryengine/v1.ts — Changed all streamAnswer
  method signatures and createAPIRequest calls from
  Schema$GoogleCloudDiscoveryengineV1AnswerQueryResponse to
  Schema$GoogleCloudDiscoveryengineV1AnswerQueryResponse[]
  (33 occurrences, 3 resource paths)

• src/apis/discoveryengine/v1beta.ts — Same changes with V1beta
  prefix (33 occurrences, 3 resource paths)

• src/apis/discoveryengine/v1alpha.ts — Same changes with V1alpha
  prefix (33 occurrences, 3 resource paths)

• src/generator/templates/method-partial.njk — Added detection
  for :stream in the method path. When the path contains :stream
  (Google's REST convention for streaming custom methods like
  :streamAnswer, :streamGenerate, etc.), the generated response type
  is automatically array-ified to prevent this bug from recurring on
  the next nightly regeneration.

Non-streaming answer methods (which correctly return a single object)
are not affected — only streamAnswer methods were changed.

Impact

• TypeScript users of @googleapis/discoveryengine can now access
  res.data[0].answerQueryToken, res.data.filter(...), etc. without
  undefined surprises or @ts-expect-error casts.
• No runtime behavior change — the REST endpoint already returned
  an array; this fix only corrects the TypeScript types to match.
• Generator fix prevents the same class of bug for any future
  :stream* methods across all APIs.

Testing

• Verified 33 occurrences of [] per version file (v1/v1beta/v1alpha),
  99 total across all streamAnswer method signatures and
  createAPIRequest calls
• Confirmed 6 answer (non-stream) method createAPIRequest calls
  remain correctly typed as single objects — no accidental changes
• Confirmed only 4 files touched, no other APIs affected
• npm run compile passes (9 pre-existing TypeScript errors in
  unrelated drive API test files exist before and after this change)

Additional Information

The discovery document itself should also be updated upstream so the
response.$ref correctly declares an array. The generator template
change in this PR acts as a safety net until that happens.

Fixes #3930 🦕

---

The top template block in your paste (with the bullet points about `streamAnswer`) would go above the checklist, roughly like:

```markdown
streamAnswer REST endpoint returns a JSON array of AnswerQueryResponse
frames at runtime, but the generated TypeScript types declared a single
object, causing res.data.answerQueryToken and similar fields to be
undefined at runtime.

- Fix v1, v1beta, and v1alpha generated types (99 total)
- Add :stream path detection in method-partial.njk generator template
  to prevent regression

Fixes #3930

Then fill the checklist as:

- [x] Make sure to open an issue as a bug/issue before writing your code
- [x] Ensure the tests and linter pass (see Testing section)
- [ ] Code coverage does not decrease (no runtime behavior change)
- [x] Appropriate docs were updated (inline JSDoc unchanged as it already
  described streaming behavior)
- [ ] Appropriate comments were added (N/A — type-only fix)
- [x] No new warnings or issues will be generated from this change

Fixes #3930 🦕

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