Add unit tests for resolveSubgraphInputLink

## Description

Add dedicated unit tests for the `resolveSubgraphInputLink` function in `src/core/graph/subgraph/resolveSubgraphInputLink.ts`.

The function has non-trivial logic (reverse iteration, link resolution, lazy widget caching) and is used in critical call sites, but currently lacks focused unit tests. The behavior is only exercised indirectly through integration-level subgraph tests.

## Test Coverage Needed

A new test file `resolveSubgraphInputLink.test.ts` should cover:
- Non-subgraph node returns `undefined`
- Missing input slot handling
- Broken link scenarios
- Lazy `getTargetWidget` caching behavior

## Context

- Related PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9282
- Original discussion: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9282#discussion_r2866949167
- Requested by: @christian-byrne

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