Deletion of messages in the frontend WEB UI

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[Contribution guide.](https://google.github.io/adk-docs/contributing-guide/)

**Is your feature request related to a problem? Please describe.**
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

**Describe the solution you'd like**
I would like to have the ability to delete messages directly in the Web UI. This could be through a delete button on each message, or via a bulk selection mode. Having this feature would keep the interface clean and focused on the most relevant content.

**Describe alternatives you've considered**
Refreshing the page, which clears the context but also removes useful messages I may want to keep.
Manually ignoring old messages, which is not practical when working with longer histories.

**Additional context**
This feature would improve usability for both developers working with the ADK Web UI. It aligns with common patterns in other conversational interfaces, where message deletion is a standard feature.