HTML widgets: which Teams clients render html-widget messages?

We'd like to use HTML widgets (#626) in a production bot and are trying to find out which Teams clients render them.

**What we sent:** `buildHtmlWidgetMessage` from `@microsoft/teams.apps` 2.0.16 (a static widget with a textarea, `domain: "https://teams.microsoft.com"`), to a personal chat with a single-tenant bot. The Bot Framework accepted it (`POST …/activities` → 201).

**What Teams showed (web client, commercial tenant):**
- Without Public preview: nothing at all — not even the `before` text.
- With Public preview enabled: the `before` text, and the ` ```html-widget ` block drawn as a plain code block labelled "Text", with the JSON payload inside.

#715 mentions verifying in the dogfood Teams web client, and the docs PR (microsoft/teams-sdk#2934) is on hold.

**Questions**
1. Which clients/rings render `html-widget` blocks today, and is Public preview on the roadmap?
2. Is anything required on the app side (manifest, tenant setting) beyond sending the message?
3. Is there a way to detect support, so a bot can fall back to an Adaptive Card where widgets are not rendered?

Start by reviewing issue #715 and the on-hold documentation PR microsoft/teams-sdk#2934, then verify the reported behavior for buildHtmlWidgetMessage across the Teams clients and preview settings named here. Done means documenting supported clients and rings, any manifest or tenant requirements, and whether support detection or an Adaptive Card fallback is available.

Written by the indexing model from the issue text.