fix(js): skip redux toggle when opening native block inserter

[jkmassel]

Summary

Tapping the + button currently calls setIsInserterOpened(true) on the editor store, which momentarily renders Gutenberg's web inserter UI before the native dialog covers it — a visible flash on slower devices. Call prepareAndShowInserter() directly instead; the redux flag isn't useful when the inserter is rendered natively.

Test plan

• On Android (or iOS), with enableNativeBlockInserter on, tap the + toolbar button
• Native inserter dialog appears immediately, no flash of the web inserter behind it
• The dialog still closes correctly via the X button (still goes through onToggle(false) from window.blockInserter.onClose)

[wpmobilebot]

XCFramework Build

This PR's XCFramework is available for testing. Add the following to your Package.swift:

.package(url: "https://github.com/wordpress-mobile/GutenbergKit", branch: "pr-build/487")

_{Built from 045288c}

[oguzkocer]

@jkmassel Do you have a recording of the flash this is fixing? Because I tried recording before and after and I don't see any change. I am working with Claude to see maybe it's just a build issue on my end because I tested the fix before I tested the base branch. Here are the recordings:

Recording from: jkmassel/android-block-picker:

487-before.webm

Recording from jkmassel/native-inserter-no-redux-flash:

487-after.webm

[oguzkocer]

I think maybe the reason I am not seeing a flash is this:

a visible flash on slower devices.

So, I am going to approve the PR on the code change and the fact that I am not observing a flash after the change. 🤷