fix(core): use the default language for a code block typed without one - #3024

Fixes #3005.

Typing ``` with no language suffix stored an empty string as the code block's
language. The input rule fell back to the raw match when there was nothing to resolve:

const languageName = match[1].trim();          // "" for a bare ```
const attributes = {
  language: getLanguageId(options, languageName) ?? languageName,   // -> ""
};

createLanguageSelect then threw Language is not supported. — the double space in the
message is the empty language — and because it threw partway through creating the block,
the block was left half-initialised, which is what produced the misplaced cursor and the
broken Backspace/Delete the reporter saw afterwards.

The fix falls back to the configured defaultLanguage ("text") when no language is
given, so the block is created with something a highlighter can actually render.

Two existing tests encoded the bug

"converts ``` + space into a codeBlock with empty language" and the + Enter equivalent
both asserted language === "". They now assert the default, and they are the regression
tests — both fail on main with expected '' to be 'text'.

One case I deliberately did not change

An unrecognised language still passes through as-is: ```notalanguage produces
language: "notalanguage", which createLanguageSelect will also reject. I left that
alone because it is a different call from the empty case — there the user typed a name and
silently replacing it with text would discard what they asked for, and a custom
highlighter registered later could legitimately support it. Happy to handle it here, or in
its own issue, if you would rather it fall back too.

Testing

packages/core: pnpm vp test --run src/blocks/Code/block.test.ts   # 19 passed
packages/core: pnpm vp test --run                                 # 759 passed, 9 skipped

pnpm vp run lint reports 49 errors both with and without this change — all
Cannot find module '@blocknote/…' from unbuilt workspace packages, none in the files
touched here.

Summary by CodeRabbit

• Bug Fixes
  • Code blocks created without a language now consistently default to text.
  • Unrecognized language suffixes fall back to the configured default language instead of producing an empty language value.