fix(ai): keep smoothed text ahead of tool execution

[iroiro147]

Summary

• wait for transformed assistant output to reach the stream boundary before executing client tools
• add regression coverage for smoothStream with a multi-step streamText tool call
• add a patch changeset for ai

Fixes #13199.

Tests

• npx --yes pnpm@10.33.4 install --frozen-lockfile
• npx --yes pnpm@10.33.4 --filter ai... build
• npx --yes pnpm@10.33.4 --dir packages/ai exec vitest --config vitest.node.config.js --run src/generate-text/stream-text.test.ts (408 passed)
• npx --yes pnpm@10.33.4 --dir packages/ai exec vitest --config vitest.node.config.js --run src/generate-text/stream-text.test.ts src/generate-text/smooth-stream.test.ts src/generate-text/execute-tools-from-stream.test.ts (458 passed)
• npx --yes pnpm@10.33.4 --dir packages/ai exec vitest --config vitest.edge.config.js --run src/generate-text/stream-text.test.ts src/generate-text/smooth-stream.test.ts src/generate-text/execute-tools-from-stream.test.ts (458 passed)
• npx --yes pnpm@10.33.4 --filter ai type-check
• npx --yes pnpm@10.33.4 exec oxfmt --check packages/ai/src/generate-text/execute-tools-from-stream.ts packages/ai/src/generate-text/stream-text.ts packages/ai/src/generate-text/stream-text.test.ts .changeset/smoothstream-tool-order.md
• npx --yes pnpm@10.33.4 exec oxlint packages/ai/src/generate-text/execute-tools-from-stream.ts packages/ai/src/generate-text/stream-text.ts packages/ai/src/generate-text/stream-text.test.ts
• git diff --check