Replace use of private HostInstance types in Fantom tests (#56947)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56947

#### Context

For the Strict TypeScript API rollout, I'm understanding how we've defined, exposed, and applied component ref/element types for built-in React Native components. These are currently inconsistent and breaking between languages (Flow, manual TS types, and generated Strict TS types) and are a key surface area for migration incompatibilities and user confusion.

As a first phase, I want to align all Flow usages towards one pattern — even if this isn't the pattern we end up shipping for TypeScript.

#### This diff

Replace use of the private `TextInputInstance` type in our Fantom test code.

- `TextInput-itest.js` now depends on public APIs only, and aligns with how external Flow callers (`xplat/js`) need to access this type.

**Notably**, this preserves `HostInstance` (`<View>`) uses (exported at root) as the remaining exception to the rule — which remains an open design problem I'm looking to address in T270727973 (via RN WG consultation).

Changelog: [Internal]

Reviewed By: christophpurrer

Differential Revision: D106128324

fbshipit-source-id: 8961dda1aab0e78a48e3824980842762df42ce38