content_type="semi-markdown" exists in the client but not in StreamingContentType #2396

Problem

The client accepts a content_type value that the Python API does not expose: "semi-markdown".

Client — shiny/www/py-shiny/markdown-stream/markdown-stream.js handles four values, dispatching semi-markdown to a different marked renderer than markdown:

if (e === "markdown") { ... renderer: uo ... }
else if (e === "semi-markdown") { ... renderer: po ... }   // distinct renderer
else if (e === "html") { ... }
else if (e === "text") { return t; }
else throw new Error(`Unknown content type: ${e}`);

Python — shinychat/_markdown_stream.py L20:

StreamingContentType = Literal[
    "markdown",
    "html",
    "text",
]

Nothing under shiny/ui/ mentions semi-markdown.

Consequence

Passing content_type="semi-markdown" works at runtime (Literal is not enforced) but fails type checking, and is undiscoverable from the Python side. The po renderer appears to be the HTML-escaping variant, which is a genuinely useful mode for streaming untrusted LLM output — so this is a real capability that is currently reachable only by accident.

Suggested resolution

Pick one:

1. Expose it — add "semi-markdown" to StreamingContentType and document what it does differently from "markdown" (escaping behavior). Preferred if the escaping mode is intentional.
2. Remove it — drop the branch from the JS if it is a vestigial port from R Shiny.

Either way the two sides should agree.

Context: the docs site briefly documented "semi-markdown" as a valid value and removed it after discovering it was absent from the Python signature (posit-dev/py-shiny-site#418). Verified against shiny 1.7.1.dev6.