fix(inline-toolbar): do not show "Convert to" when there is nothing to convert to - #3040

Summary

The inline toolbar shows a Convert to item even when there is nothing the current block can be converted to — it opens an empty submenu. As confirmed in #2833: "We should not show convertTo if there is no blocks to convert to."

This PR fixes that for the inline toolbar. (It does not make convertTo configurable via inlineToolbar — per the discussion in #2833 that part is not planned.)

Root cause

getConvertibleToolsForBlock() filters each candidate tool's toolbox entries (dropping entries that describe the current block itself, entries without icons, etc.) but then keeps the tool even when all of its entries were filtered out, leaving it with an empty toolbox.

Example: in an editor with only the default paragraph tool, the paragraph's single toolbox entry is filtered out (it describes the current block), yet the tool was still returned as "convertible".

ConvertInlineTool.render() only checked the number of convertible tools, not the number of resulting menu items, so it returned a Convert to item whose items list is empty. The block tunes menu (blockSettings.ts) already guards on the items count, which is why the bug only appeared in the inline toolbar.

Changes

• src/components/utils/blocks.ts — getConvertibleToolsForBlock() now skips tools whose toolbox entries were all filtered out.
• src/components/inline-tools/inline-tool-convert.ts — render() returns no item when there are no convert items, mirroring the existing guard in the block tunes menu.
• test/cypress/tests/modules/InlineToolbar.cy.ts — regression test: with only the default paragraph tool available, selecting text opens the inline toolbar without a convert-to item.

Related issue

#2833 (the maintainer-confirmed part: Convert to must not be shown when there are no blocks to convert to)

Testing

• eslint on all changed files: no errors. Full eslint src/ totals are identical before and after this change (257 problems — 23 errors / 234 warnings — pre-existing on next with the locally resolved toolchain).
• vite build (production): passes.
• vite build --mode test (build:test): passes.
• New Cypress regression test added (see above). I could not run Cypress locally — the Cypress binary download does not complete in my sandbox — so the Cypress suite result is left to CI here; the test follows the existing patterns in the same spec file. The reproduction itself was verified against the published @editorjs/paragraph package (single toolbox entry without data, conversionConfig import/export text), which is exactly the case that produced an empty Convert to submenu.