[fix]: import ToolSet from ai public export (browserbase#2126)

thanks @asim48-ctrl for the contribution!

## Summary
- replace the remaining `ai/dist` deep import with the public `ai`
package export
- aligns `AgentProvider` with the other Stagehand v3 CUA files that
already import `ToolSet` from `ai`

Fixes browserbase#1531.

## Verification
- `corepack pnpm install --ignore-scripts --frozen-lockfile`
- `corepack pnpm --filter @browserbasehq/stagehand run gen-version`
- `corepack pnpm --filter @browserbasehq/stagehand run
build-dom-scripts`
- `corepack pnpm exec tsc -p packages/core/tsconfig.json --noEmit`
- `corepack pnpm exec prettier --check
packages/core/lib/v3/agent/AgentProvider.ts`
- `git diff --check`

<!-- This is an auto-generated description by cubic. --> ---
## Summary by cubic
Switch AgentProvider to import ToolSet from the public `ai` export
instead of `ai/dist` to prevent deep import breakage. Adds a patch
changeset and aligns with other Stagehand v3 CUA files; fixes browserbase#1531.

<sup>Written for commit 1c17675.
Summary will update on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

Co-authored-by: asim48-ctrl <asim48@gmail.com>

Author: seanmcguire12

.changeset/light-books-dance.md

@@ -0,0 +1,5 @@
+---
+"@browserbasehq/stagehand": patch
+---
+
+fix(core): import ToolSet from ai public export

packages/core/lib/v3/agent/AgentProvider.ts

@@ -1,4 +1,4 @@
-import { ToolSet } from "ai/dist";
+import { ToolSet } from "ai";
 import { AgentProviderType } from "../types/public/agent.js";
 import { LogLine } from "../types/public/logs.js";
 import { ClientOptions } from "../types/public/model.js";