Add append_text_example docs snippet

[techwritermat]

Summary

Adds the append_text_example snippet that the Unity Chat docs reference (drafts page). Without it the docs render 'Code snippet not found'.

Docs PR: https://github.com/PubNubDevelopers/documentation/pull/1023

Test plan

• Method compiled against PubnubChatApi MessageDraft (net9.0 console project), 0 errors
• Not built inside the Unity editor

🤖 Generated with Claude Code

[pubnub-snyk-universal-prd]

✅ Snyk checks have passed. No issues have been found so far.

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[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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[jakub-grzesiowski]

LGTM