Fix multiple item types list snippet

[nero002]

• Render only the message for the current LazyColumn item
• Use the message class as contentType so items of the same type can reuse compositions
• Fixes ListWithMultipleItemTypes.kt — Incorrect usage of items() lambda and contentType #863

This corrects the multiple item types snippet used by the Compose quick guide.

Test: ANDROID_HOME=/Users/Dheeraj/Library/Android/sdk ./gradlew :compose:snippets:compileDebugKotlin

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[gemini-code-assist]

Code Review

This pull request refactors a LazyColumn implementation to correctly handle multiple item types by removing an erroneous internal loop and utilizing the item index. It also updates the contentType parameter to use the message class for better performance. A review comment identifies the use of org.w3c.dom.Text as inappropriate for Android development and suggests refactoring to use a String or a custom data class.