fix(apps): source informative textFormat from the update + add textFormat to update()

[Kavin (singhk97)]

Summary

Follow-up to #762. Two fixes for textFormat on streamed informative updates:

1. Source informative textFormat from the update itself. In HttpStream.flush(), informative typing chunks previously took their format from this.finalActivity?.textFormat. But finalActivity is only set once a message is drained from the queue — which happens after informative updates are collected — so an informative update's own textFormat was ignored. It now reads informativeUpdate.textFormat directly. The streamed-text chunk keeps its correct last-emitted-message-wins behavior.

2. update() can now carry a textFormat. IStreamer.update(text) / HttpStream.update(text) gain an optional, nullable textFormat:

   update(text: string, textFormat?: TextFormat | null): void;

   undefined/null → Teams default (markdown); an explicit value (e.g. extendedmarkdown) → that format. Previously the only way to format an informative update was to hand-build a { type: 'typing', ..., textFormat } object and call emit().

Sample

examples/stream demonstrates the new update() overload and cleans up the extended-markdown scenario's deltas to match the existing sample style while showcasing checkboxes and strikethrough.

Tests

Added to http-stream.spec.ts:

• informative update carries its own textFormat, independent of finalActivity (red/green — fails against the old finalActivity-sourced logic)
• update(text, 'extendedmarkdown') sends an informative chunk with that format
• update(text) and update(text, null) omit textFormat

All apps tests pass (648/648), build + eslint clean.

Cross-language

Counterpart PRs (not yet merged) will fold in the equivalent changes:

• Python Preserve text_format through HttpStream's intermediate and final activities teams.py#581
• .NET Preserve textFormat through TeamsStreamingWriter's intermediate and final activities teams.net#661 (design differs — coordinated separately)

[Copilot]

Pull request overview

This PR fixes how textFormat is handled for streamed informative typing updates in HttpStream, and extends the streaming API so update() can explicitly control formatting without requiring callers to hand-build a typing activity.

Changes:

• Fix HttpStream.flush() to source informative update textFormat from the informative update itself (instead of finalActivity).
• Extend IStreamer.update() / HttpStream.update() to accept an optional, nullable textFormat.
• Add unit tests covering informative textFormat behavior and the new update() overload; update the streaming example to use the overload.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
packages/apps/src/types/streamer.ts	Extends the IStreamer.update() signature and documents textFormat semantics.
packages/apps/src/http/http-stream.ts	Implements update(text, textFormat?) and fixes informative update textFormat sourcing in flush().
packages/apps/src/http/http-stream.spec.ts	Adds tests for informative textFormat and the new update() overload behavior.
examples/stream/src/index.ts	Updates the extended-markdown sample to use the new update() overload and refreshes sample deltas.

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