feat: Support viewport history navigation while streaming

[liaowang11]

When a user switches the viewport to view previous interactions while a response is actively streaming, we should support navigating the history without interrupting the stream.

I find this is most useful when preparing for followup questions while the agents are busy working.

This change implements the following:

1. Adds agent-shell-viewport--showing-latest-p to check if the viewport is on the latest interaction.
2. Skips echoing stream updates (text, fragments) to the viewport buffer if it is currently viewing a historical interaction.
3. Allows agent-shell-viewport-next-page to be used while the agent is busy.

Related: #433

Checklist

• I agree to communicate (PR description and comments) with the author myself (not AI-generated).
• I've reviewed all code in PR myself and will vouch for its quality.
• I've read and followed the Contributing guidelines.
• I've filed a feature request/discussion for a new feature.
• I've added tests where applicable.
• I've updated documentation where necessary.
• I've run M-x checkdoc and M-x byte-compile-file.

Verification Notes

• The viewport-history tests added in this branch pass.
• checkdoc reports one existing warning at agent-shell.el:2373.
• A full batch ERT run in this environment still has two unrelated failures: agent-shell--build-content-blocks-binary-file-test and agent-shell-restart-preserves-default-directory.