Add Playwright test steps to nested subgraph value-preservation regression

## Summary
Add Playwright `test.step()` calls to the nested-subgraph promoted-value regression test so the user-flow phases are visible in Playwright/CI logs.

## Rationale
The test currently communicates its flow primarily through comments. Named steps will make failures easier to diagnose by identifying whether setup, widget promotion/editing, nested conversion, or final verification failed.

## Affected area
- `browser_tests/tests/subgraph/subgraphNestedConversionValuePreservation.spec.ts`

## Required changes
Group the existing end-to-end flow into clear, meaningful `test.step()` blocks without changing the behavior covered by the regression test. Preserve the existing assertions and follow the repository's Playwright testing guidance.

## Acceptance criteria
- The test uses descriptive `test.step()` blocks for the major user-flow phases.
- A failed run identifies the relevant phase in Playwright output.
- The existing regression coverage and assertions remain intact.

## Backlinks
- Pull request: https://github.com/Comfy-Org/ComfyUI_frontend/pull/13860
- Review comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/13860#discussion_r3625673658
- Requested by: @christian-byrne