test(otel): cover interleaved generate tool spans

[iroiro147]

Summary

• add OpenTelemetry regression coverage for interleaved generateText call lifecycles with tool spans
• assert each chat/tool span keeps the correct per-call step context even when events for two call IDs are interleaved
• assert span attributes remain scoped to the correct call ID and root function ID

Refs #8897.

Tests

• npx --yes pnpm@10.33.4 --filter @ai-sdk/otel... build
• npx --yes pnpm@10.33.4 --filter @ai-sdk/otel test -- open-telemetry.test.ts (197 passed)
• npx --yes pnpm@10.33.4 --filter @ai-sdk/otel type-check
• npx --yes pnpm@10.33.4 exec oxfmt --check packages/otel/src/open-telemetry.test.ts
• npx --yes pnpm@10.33.4 exec oxlint packages/otel/src/open-telemetry.test.ts
• git diff --check