Per project convention (#2735), C# handler lambdas take
(context, cancellationToken) and thread the token through Send,
Task.Delay, and SDK calls. The reactions guide didn't follow this.

Uses named-arg form (cancellationToken: cancellationToken) for the
reaction API calls because AddAsync / DeleteAsync have an optional
AgenticIdentity parameter sitting between reactionType and the token.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Per project convention (#2735), C# handler lambdas take
(context, cancellationToken) and thread the token through Send,
Task.Delay, and SDK calls. The reactions guide didn't follow this.

Uses named-arg form (cancellationToken: cancellationToken) for the
reaction API calls because AddAsync / DeleteAsync have an optional
AgenticIdentity parameter sitting between reactionType and the token.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Per project convention (#2735), C# handler lambdas take
(context, cancellationToken) and thread the token through Send,
Task.Delay, and SDK calls. The reactions guide didn't follow this.

Uses named-arg form (cancellationToken: cancellationToken) for the
reaction API calls because AddAsync / DeleteAsync have an optional
AgenticIdentity parameter sitting between reactionType and the token.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Per project convention (#2735), C# handler lambdas take
(context, cancellationToken) and thread the token through Send,
Task.Delay, and SDK calls. The reactions guide didn't follow this.

Uses named-arg form (cancellationToken: cancellationToken) for the
reaction API calls because AddAsync / DeleteAsync have an optional
AgenticIdentity parameter sitting between reactionType and the token.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

## Summary

Adds an in-depth guide for message reactions covering outbound
add/remove and inbound `messageReaction` handling, plus links the
existing "Reactions" section in the Sending Messages essentials page to
the new guide.

## What's covered in the guide

- Public-developer-preview banner
- Adding and removing reactions (`api.reactions.add` /
`api.reactions.delete` / `Reactions.AddAsync` / `Reactions.DeleteAsync`)
- Receiving reactions: `messageReaction` activity handlers across all
three SDKs, plus the .NET-only `OnMessageReactionAdded` /
`OnMessageReactionRemoved` sub-handlers
- Available reaction types — the modern set of 6 (`like`, `heart`,
`1f440_eyes`, `2705_whiteheavycheckmark`, `launch`, `1f4cc_pushpin`)
- Skin tone variants (`-tone1` … `-tone5`)
- Rate limits (2 reactions/sec, 429 + Retry-After)
- Best practices and differences from Feedback

## What's intentionally omitted

- **Custom reactions section** — aligned with the Microsoft Learn agent
reactions PR
([msteams-docs#13991](MicrosoftDocs/msteams-docs#13991)),
which is omitting custom reaction examples due to a known client-side
rendering issue. A commented-out link to the Learn reactions reference
remains in the source for restoration once that PR lands.
- **Advanced API client usage** — the previous draft's example
referenced an `ApiClient` constructor that doesn't exist in the current
SDK; removed rather than rewritten.

## Coordination

- C# samples follow the project convention from #2735 (cancellationToken
in handler signatures, threaded through `Send`, `Task.Delay`, and
reaction API calls).
- The TypeScript include uses `api.reactions.delete()` ahead of
[microsoft/teams.ts#567](microsoft/teams.ts#567),
which renames `remove()` → `delete()` to align with teams.py and
teams.net.

## Test plan

- [x] `npm start` regenerates docs cleanly with no content gaps
- [x] Per-language renders (TS / Python / C#) verified in the dev server
- [x] Cross-link from `essentials/sending-messages` → in-depth guide
resolves correctly
- [ ] Visual review on the deployed preview

---------

Co-authored-by: Corina Gum <[email protected]>
Co-authored-by: Claude Opus 4.7 (1M context) <[email protected]>