fix(xai): preserve web_search action (query, sources, open_page) in responses tool results (#19796)

## Background

Reported by AthenaHQ ([customer
thread](https://vercel.slack.com/archives/C09Q2N54LJC/p1787771675800939)):
calling xAI's `web_search` tool through the AI Gateway Responses API
returns `web_search_call` items with an empty `action` — `query` is
`''`, no `sources`, no `open_page` items. Direct to
`api.x.ai/v1/responses` the same request returns everything (8 search
calls, 35 source URLs, 5 opened pages in my repro).

Root cause: the xAI responses language model drops `part.action` for
`web_search_call` output items. `doGenerate` emitted only a `tool-call`
with no `tool-result`, and `doStream` emitted `tool-result` with
`result: {}`. Downstream consumers (the gateway's Responses
normalization) then fall back to `{type: 'search', query: ''}`.

## Summary

- Emit a `tool-result` for `web_search_call` parts in both `doGenerate`
and `doStream`, carrying the mapped action via a new
`mapWebSearchAction` helper that mirrors `@ai-sdk/openai`'s
`mapWebSearchOutput`: `search` (query/queries + sources), `open_page` →
`openPage`, `find_in_page` → `findInPage`
- Update the `webSearch` tool output schema to the Responses-style shape
(`action` + `sources`) so the tool output is typed
- Tests for search/open_page/find_in_page action mapping in generate and
stream

## Verification

- `vitest run` in packages/xai: 97/97 pass
- End-to-end through a local AI Gateway with this build patched in as
`@ai-sdk/xai-v3` plus the companion gateway change: 9 web_search_call
items, 40 source URLs, 5 open_page actions (was: 8 items, all empty)

Companion gateway PR forwards `sources` in the Responses output mapper.
Linear:
[AIG-670](https://linear.app/vercel/issue/AIG-670/xai-web-search-call-items-lose-query-sources-and-open-page-actions)

The customer-facing gateway path runs the v3 (release-v6.0) line today,
so this should be backported.