fix: conditionally set citation encodingFormat based on text presence

[chonlaphoom]

While following the citations example (https://microsoft.github.io/teams-sdk/typescript/in-depth-guides/ai/best-practices) (which calls addCitation without text), I encountered a BadSyntax error: citation.appearance.text and citation.appearance.encodingFormat must either both be empty or both have values. This happens because encodingFormat was unconditionally set to 'application/vnd.microsoft.card.adaptive', even when text was omitted

Summary

Fixes BadSyntax error when addCitation is called without text.

Problem

Calling addCitation(position, appearance) without appearance.text causes a 400 error:

code: "BadSyntax"
message: "Error while parsing citation entity with id: x"
Detail: "citation.appearance.text and citation.appearance.encodingFormat must either both be empty or both have values"

The root cause: encodingFormat was hardcoded to 'application/vnd.microsoft.card.adaptive' unconditionally, but the Teams API requires it to be absent when text is absent.

Fix

Changed encodingFormat to be set only when appearance.text is truthy:

encodingFormat: appearance.text ? 'application/vnd.microsoft.card.adaptive' : undefined,

[Copilot]

Pull request overview

This PR fixes citation payload construction so encodingFormat is only populated when citation text is present, avoiding Teams API BadSyntax responses for citations without adaptive card text.

Changes:

• Conditionally sets citation encodingFormat based on appearance.text.
• Updates addCitation tests to cover text/no-text behavior and icon handling.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/api/src/activities/activity.ts	Updates citation appearance construction to avoid setting encodingFormat without text.
packages/api/src/activities/activity.spec.ts	Adjusts and expands addCitation test cases for encoding format behavior.

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