Terminal block hover overlay

[Fangoling]

What it does

Adds experimental terminal block hover actions on top of command history.

When terminal.integrated.enableCommandHistory and the new terminal.integrated.enableHoverActions preference are enabled, executed command blocks get a hover affordance that opens a block action menu. The menu lets users copy the command, copy the output, copy both, and scroll to the top or bottom of the block.

For AI-enabled setups, this also adds an Ask AI about Command action for terminal blocks with command/output content. That action opens an inline Ask AI overlay in the terminal, submits the selected block context to a new chat session, and includes cleanup fixes for the overlay, widget disposal, and child DI container lifecycle.
When #17312 is merged, we can also pass in the terminal AI Variable instead of the command content as plain text.

The implementation also places the hover overlay logic into a dedicated TerminalBlockHoverOverlayController to keep marker tracking, viewport updates, and disposal isolated.

The hover effect only appears when hovering over the ellipsis button at the far right of each command block, this is a limitation since the hover effect would conflict with the existing terminal hover links.

How to test

1. Start Theia with AI Terminal available.
2. Open Settings and enable:
   • terminal.integrated.enableCommandHistory
   • terminal.integrated.enableHoverActions
3. Open a user terminal and run a few commands that produce visible output, for example:
   • echo hello
   • ls
   • pwd
4. Hover over the action button of a completed command blocks and verify the hover action button appears.
5. Open the block menu and verify:
   • Copy Command and Output copies both values
   • Copy Command copies only the command
   • Copy Output copies only the output
   • Scroll to Top of Block and Scroll to Bottom of Block move the viewport to the expected location
6. With AI enabled, verify Ask AI about Command appears for blocks that have command and output.
7. Trigger Ask AI about Command, confirm the inline overlay appears in the terminal, submit a prompt, and verify a chat session opens with the terminal command/output prepended to the request.
8. Reopen the overlay, cancel it, and close the terminal while the overlay is open to confirm there are no disposal errors or duplicate-dispose issues.

Follow-ups

• Add automated frontend coverage for terminal block hover actions and overlay lifecycle/disposal behavior.
• Evaluate whether the Ask AI overlay should support richer block formatting or block truncation for very large outputs.
• Consider additional terminal block actions if this experimental UI proves useful.

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines
• User-facing text is internationalized using the nls service (for details, please see the Internationalization/Localization section in the Coding Guidelines)

Reminder for reviewers

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[sdirix]

Generally it works for me.

The name "Enable Hover Actions" seems off as the ... button is always shown, not only on hover. So maybe it should be called "Enable Command Block Actions"? Or we should show the ... buttons only on hover, for now they are always shown for me.

When enabling the preference, the first ... button is only shown for the 2nd next block, not the next block. Can this be improved?

When disabling the preference and enabling it again, the ... buttons are not shown for blocks for which they were shown in the past. Can this be improved?