feat: add literalTagContent prop for plain-text custom tag children

[sleitor]

Summary

Adds an opt-in literalTagContent prop that renders children of specified custom tags as plain text (no markdown parsing). This is useful for mention/entity tags in AI UIs where child content is a data label (e.g. a username or handle) rather than prose.

Motivation

When using allowedTags with custom components, markdown inside tag children is still parsed. For example:

<mention user_id="123">@_some_username_</mention>

renders with emphasis on _some_username_, which is unintended for entity tags.

Requiring model-side escaping (\_) everywhere is fragile and adds prompt/runtime complexity.

Solution

A new literalTagContent?: string[] prop. For tags in that list, markdown metacharacters inside the tag content are escaped before the markdown parser runs, so the parser treats the children as plain text.

<Streamdown
  allowedTags={{ mention: ['user_id'] }}
  literalTagContent={['mention']}
>
  {`<mention user_id="123">@_some_username_</mention>`}
</Streamdown>

Output: renders @_some_username_ as literal text — no emphasis.

Why preprocessing, not a rehype plugin

The earlier approach (a rehype plugin that flattens children to text post-parse) loses the original characters — by the time rehype runs, _some_username_ has already been parsed into <em>some_username</em> so the underscores are gone. Escaping before parsing preserves the original content.

Changes

• New packages/streamdown/lib/preprocess-literal-tag-content.ts — escapes markdown metacharacters inside matching tag content before parsing
• Modified packages/streamdown/index.tsx — adds literalTagContent?: string[] to StreamdownProps and wires the preprocessor into processedChildren
• Modified packages/streamdown/__tests__/allowed-tags.test.tsx — 5 new tests for the literalTagContent prop
• New packages/streamdown/__tests__/preprocess-literal-tag-content.test.ts — 10 unit tests for the preprocessor

All 807 existing tests continue to pass.

Closes #419

[vercel]

Someone is attempting to deploy a commit to the Vercel Team on Vercel.

A member of the Team first needs to authorize it.

[vercel]

Additional Suggestion:

When a tag appears in both allowedTags and literalTagContent and its content contains blank lines, the <!----> HTML comment inserted by preprocessCustomTags gets corrupted to <\!\-\-\-\-> by the subsequent preprocessLiteralTagContent call.

[sleitor]

Thanks for the automated review! The HTML comment corruption issue is fixed in the latest commit (465de11).

Root cause: preprocessCustomTags ran before preprocessLiteralTagContent. When a tag appeared in both lists and its content contained blank lines, the <!----> markers inserted by preprocessCustomTags were subsequently escaped to <\!\-\-\-\-> by preprocessLiteralTagContent (which treats ! and - as markdown metacharacters).

Fix: Swapped the execution order — preprocessLiteralTagContent now runs first, so the markdown escaping happens before the blank-line markers are inserted. The <!----> comments are never seen by the escaper.