Forms: fix block inserter collapsing in Firefox

[monsieur-z]

Fixes https://github.com/Automattic/jetpack/issues/39144

Proposed changes:

This draft PR explores fixing the issue linked above: in the latest versions of Firefox (reported in 129 and 128), the block inserter collapses when various actions are operated. For instance:

• when the user starts typing in the search box
• when the cursor hovers any Jetpack Forms block

This only happens with Jetpack Forms blocks. If we don't register those blocks, the inserter works seamlessly.

Block inserter

It's been hard to reproduce the issue consistently every time. At times, the inserter collapses immediately. Other times, it takes 5 to 10 times hovering it. No error is logged in the console.

Other information:

• [ ] Have you written new tests for your changes, if applicable?
• [ ] Have you checked the E2E test CI results, and verified that your changes do not break them?
• [ ] Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

n/a

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

• Spin a test site with this branch
• Create a new post
• Open the block inserter by clicking the + button
• Ensure the search functionality works and the inserter does not collapse
• Hover various forms blocks: the inserter should not collapse
• Try hovering several times and at various speeds