Remove Zero Width Spaces on MDAST operations

[Soxasora]

Description

Fixes #2717
Removes ​/\u200b that gets injected by some browsers (Chromium).

We don't generate the ZWS in our editor, but can be copied and pasted from somewhere else. Since this character can spawn in other circumstances (HTML import, node replacements, some Android IMEs), the most effective place to remove this character is inside the markdown strings we give to the MDAST pipelines (export/import).

Screenshots

tbd

Additional Context

Removing the ZWS inside the editor, while editing, involves selection heuristics that we're better off not touching. Lexical uses it on any device that's not Apple.

This character can't spawn unwillingly on iOS devices, it's a Chrom(ium) thing afaict

Checklist

Are your changes backward compatible? Please answer below:

For example, a change is not backward compatible if you removed a GraphQL field or dropped a database column.
Yes
On a scale of 1-10 how well and how have you QA'd this change and any features it might affect? Please answer below:
7, touches only textnode visitors

For frontend changes: Tested on mobile, light and dark mode? Please answer below:
Not a frontend change, but I've tested this on Android which is the most impacted platform.

Did you introduce any new environment variables? If so, call them out explicitly here:
n/a

Did you use AI for this? If so, how much did it assist you?
No

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Note

Ensures markdown passed through Lexical and MDAST is free of zero‑width spaces that some browsers inject.

• Add removeZeroWidthSpace and apply it in $getMarkdown, markdownToLexical (pre-import), and lexicalToMarkdown (post-export)
• Update FormikBridgePlugin to use $getMarkdown() for syncing, instead of raw getTextContent()
• Keep empty-state handling via $isMarkdownEmpty() unchanged

^{Written by Cursor Bugbot for commit ac64e59. This will update automatically on new commits. Configure here.}

[Soxasora]

I'm not sure this'll cover every edge case since parsing can fail with ZWS. So I'll add a comment to address this when we'll reach the Rich Text stage