Fix streaming behavior by capturing typingActivityId in nextActivityIdToKeyMap

[benbrown]

Fixes #5514

Description

This fixes the streaming behavior where new interim messages will replace the old, appear to append the message as it streams in. This is a follow-up to #5517 which initially introduced support for parsing and processing those entities.

However, this small additional change was required for the rendering process to properly find the existing message and update it rather than insert a new message.

Specific Changes

This fix was achieved by capturing the typingActivityId along with the main activityId in the nextActivityToId map such that it can be found the second time around.

In addition, added html2/livestream/entities.*, a selection of tests that demonstrate that the behavior with entities matches the behavior with channelData.

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• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

[compulim]

Commented, also few things need to be answered by Eugene.