[lexical] Fix: preserve block equations ($$) in MarkdownShortcutPlugin

[Jynx2004]

Description

Block math equations ($$...$$) created via the toolbar were incorrectly converted to inline equations ($...$) when toggling markdown mode via the MarkdownShortcutPlugin.

Root cause: The EQUATION transformer's export function always returned $${equation}$ (inline syntax) regardless of whether the node was a block or inline equation. There was also no transformer to import/match $$...$$ patterns.

Closes #6936

Test plan

Created a new test for testing this functionality .

Before

inserting a block equation and clicking the Markdown button converted $$x^2 + y^2 = z^2$$ → $x^2 + y^2 = z^2$ (inline)

After

block equations are preserved as $$x^2 + y^2 = z^2$$ when toggling markdown mode

[vercel]

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Project	Deployment	Actions	Updated (UTC)
lexical	Ready	Preview, Comment	Mar 15, 2026 5:41pm
lexical-playground	Ready	Preview, Comment	Mar 15, 2026 5:41pm

[etrepum]

This only handles export, not import, so it's not complete.

[etrepum]

See also #8194 which is probably a more complete fix but is not yet acceptable because there are no tests.

[Jynx2004]

This only handles export, not import, so it's not complete.

I think having a single import for both inline as well as block case will also work fine as both have the same trigger $

[etrepum]

It could possibly be implemented in one transformer or two, but either way this PR is not complete because it doesn’t have test coverage to show that import works. When you write those tests you’ll see that it probably doesn’t work as-is.

[Jynx2004]

It could possibly be implemented in one transformer or two, but either way this PR is not complete because it doesn’t have test coverage to show that import works. When you write those tests you’ll see that it probably doesn’t work as-is.

Made some changes in the unit test to check the import works for both the cases .

[etrepum]

What's needed to demonstrate that this PR is complete are tests to ensure that the exported markdown can be imported and the result is an EquationNode with the right setting for inline and the correct contents.