refactor: remove internal viewportCache exposure from subgraphNavigationStore and test through public interfaces

## Context

In PR #10247, `viewportCache` was exposed on the public return of `useSubgraphNavigationStore` and marked `@internal` as a pragmatic workaround so unit tests in `src/stores/subgraphNavigationStore.viewport.test.ts` could make direct assertions on the LRU cache (e.g., `viewportCache.set/get/has/delete`).

Exposing the raw LRU gives any consumer full read/write/delete access to internal state, which undermines the store's encapsulation boundary.

## Goal

- Remove `viewportCache` from the store's public return.
- Refactor `src/stores/subgraphNavigationStore.viewport.test.ts` to test viewport behavior exclusively through the public API (`saveViewport`, `restoreViewport`, `saveCurrentViewport`) — e.g., by asserting on observable side-effects such as `canvas.ds.scale`, `canvas.ds.offset`, `canvas.setDirty` calls, and `fitView` spy invocations — rather than inspecting cache internals directly.

## References

- PR: #10247
- Comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/10247#discussion_r2998523932

Requested by @DrJKL.

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