fix(amazon-bedrock): resolve globalThis.fetch lazily to support telemetry and fetch patching

[heiwen]

Background

createAmazonBedrock() calls either createSigV4FetchFunction or createApiKeyFetchFunction at provider initialization time. Both functions accepted the underlying fetch implementation via a default parameter:

```ts
fetch: FetchFunction = globalThis.fetch
```

JavaScript default parameters are evaluated at call time — meaning `globalThis.fetch` is read and captured the moment `createAmazonBedrock()` is called. Any patch to `globalThis.fetch` applied after that point is silently ignored for all Bedrock requests.

The most common real-world impact is telemetry instrumentation (e.g. OpenTelemetry, Datadog, custom tracing), which typically patches `globalThis.fetch` at startup to intercept outgoing HTTP requests. Because Bedrock snapshots fetch before those patches are applied, all Bedrock requests bypass instrumentation. Test setups that patch `globalThis.fetch` after imports are equally affected.

Every other provider in the SDK avoids this by deferring the `globalThis.fetch` lookup to request time. The comment in `http-chat-transport.ts` captures the intent explicitly: "avoid caching globalThis.fetch in case it is patched by other libraries".

Summary

This PR makes `createSigV4FetchFunction` and `createApiKeyFetchFunction` resolve `globalThis.fetch` lazily inside the returned closure instead of at function call time.

Changes included:

• Removed the `= globalThis.fetch` default parameter from both functions, making `fetch` optional
• Added `const effectiveFetch = fetch ?? globalThis.fetch` inside each returned closure, so the lookup happens at request time
• Added a patch changeset for `@ai-sdk/amazon-bedrock`

Manual Verification

I manually verified by inspecting the call path from `createAmazonBedrock()` through to the SigV4 and API key fetch wrappers:

• confirmed the default parameter no longer captures `globalThis.fetch` at provider initialization
• confirmed each request-time closure now resolves `globalThis.fetch` fresh on every call, consistent with how `post-to-api.ts` and `http-chat-transport.ts` handle this

Checklist

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

Related Issue

Fixes: #14364

[aayush-kapoor]

change looks good but how did you verify the change manually? + can you simulate it using unit tests? if so, please add them.

also please sign your commits for this PR to be merged

[heiwen]

Merge conflict solved, commits signed, unit test added.

For manual verification, you can use this snippet:

const { bedrock } = await import('@ai-sdk/amazon-bedrock');

const originalFetch = globalThis.fetch;

globalThis.fetch = async (...args) => {
  console.log('wrapped fetch called');
  return originalFetch(...args);
};

await bedrock('amazon.nova-lite-v1:0').doGenerate({
  prompt: [
    {
      role: 'user',
      content: [{ type: 'text', text: 'hello' }],
    },
  ],
});

// console should show "wrapped fetch called"

[aayush-kapoor]

please fix the merge conflicts and i'll get this merged

[heiwen]

Merge conflicts solved

[github-actions]

⚠️ Backport to release-v6.0 created but has conflicts: #14914