fix(google): recognize Google Files URLs when using a baseUrl (#19868)

## Background

AI SDK uses a model's `supportedUrls` to decide whether a file URL can
be passed directly to the provider or must be downloaded first.

The Google provider constructed its `supportedUrls` Google Files URL
from the configured `baseURL`. When `baseURL` points to a
Google-compatible proxy, Google Files URLs such as:

`https://generativelanguage.googleapis.com/v1beta/files/...`

do not match the proxy URL.

Core therefore treats the file as unsupported and attempts to download
it. Google Files URIs are provider resource references intended to be
passed to Gemini as `fileData.fileUri`, not public content URLs that can
be downloaded. The generic downloader does not authenticate as the
Google provider, so the request fails.

## Summary

Always include the default Google Files URL pattern in `supportedUrls`.

## End-to-End Verification

Verified against the live Gemini API using a local forwarding proxy:

1. Uploaded an MP3 directly through Google's Files API, which returned a
canonical `https://generativelanguage.googleapis.com/v1beta/files/...`
URI.
2. Created a separate Google provider with its `baseURL` pointing to the
local proxy.
3. Passed the canonical Google Files URI as an `audio/mpeg` file part
4. Confirmed that the proxy received only the `generateContent` request
and no file download request.
5. The proxy forwarded the request to Google, and Gemini successfully
processed the uploaded audio

## Checklist

- [x] All commits are signed (PRs with unsigned commits cannot be
merged)
- [x] Tests have been added / updated (for bug fixes / features)
- [] Documentation has been added / updated
- [x] A *patch* changeset for relevant packages has been added (for bug
fixes / features - run `pnpm changeset` in the project root)
- [x] I have reviewed this pull request (self-review)

## Related Issues

Fixes https://github.com/vercel/ai/issues/7796

Closes https://github.com/vercel/ai/pull/8776