Paste: Don't convert LaTeX to a Math block when it can't be inserted - #84059

What?

Closes #84054

Stops pasted LaTeX from being converted into a Math block when the Math block can't be inserted, so the text is pasted normally instead of being silently dropped.

Why?

When a user disables the Math block (Preferences → Blocks → Manage block visibility), pasting a formula like x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} into a paragraph pastes nothing.

pasteHandler converts plain text that looks like LaTeX into a core/math block without checking whether that block is allowed. Disabling a block in Preferences doesn't unregister it. It only removes it from the allowed block types, so the editor then refuses to insert the generated block and the paste is lost.

How?

• pasteHandler (@wordpress/blocks) gets a new optional convertLatexToMath option, which defaults to true, so existing callers are unaffected. It also checks that core/math is registered before converting.
• The two paste call sites in @wordpress/block-editor (rich-text/event-listeners/paste-handler.js and writing-flow/use-clipboard-handler.js) pass canInsertBlockType( 'core/math', rootClientId ). This keeps @wordpress/blocks free of editor-store knowledge. It also covers parents that don't allow the Math block, not just the Preferences toggle.
• When conversion is skipped, the text falls through to the normal plain-text/markdown path.
• Added a unit test, a changelog entry, and the regenerated packages/blocks/README.md for the new option.

This doesn't cover the other points raised in the issue: pasting a formula into a list as inline math, or a setting to turn off auto-conversion while Math stays enabled. Copying from a source that puts <pre>/styled HTML on the clipboard (for example VS Code) still produces a Code block, as before.

Testing Instructions

1. Open a post and insert a paragraph.
2. Copy this formula from a plain-text source (not a code editor): x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}
3. Paste it into an empty paragraph. A Math block is created.
4. Undo, then open Options → Preferences → Blocks, and uncheck Math.
5. Paste the formula into an empty paragraph again. It is pasted as plain text in the paragraph.
6. Re-check Math and confirm step 3 works again.
7. Run npm run test:unit -- packages/blocks/src/api/raw-handling.

Testing Instructions for Keyboard

1. Use Tab to move focus into the paragraph, then paste with Cmd/Ctrl+V.
2. Open Preferences with Shift+Alt+Cmd/Ctrl+P (or Options menu → Preferences), then Tab to the Blocks tab and toggle the Math checkbox with Space.
3. Confirm the same results as above.

Screenshots