feat: implement terminal command AI Variable

[Fangoling]

What it does

This PR builds on top of #16732.
This PR introduces a new AI Variable that allows the user to target a specific command in their last used terminal.

• the user can specify the command using an index directly or via the arguments picker
• If no index is provided, the variable defaults to the last executed command

If the experimental command history preference is disabled, this ai variable will return the last 50 lines of the terminal buffer.

ai-variable-terminal-1775517866896.mp4

How to test

1. Open the Theia Settings
2. Enable Terminal › Integrated: Enable Command History
3. Execute some commands in the terminal
4. Open the AI Chat
5. Use the #terminalCommand AI Variable in the AI Chat and choose a terminal command via the arguments picker
6. Prompt the agent to repeat the content of the prompt to verify that the correct command was passed

Follow-ups

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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[eneufeld]

What is the use case for this, especially with the variable?
Wouldn't it be better to have a toolcall? then I understand that the llm could decide itself that it needs to check a command that ran in the past and its output.

[Fangoling]

What is the use case for this, especially with the variable? Wouldn't it be better to have a toolcall? then I understand that the llm could decide itself that it needs to check a command that ran in the past and its output.

This is a building block for #17308 which adds per-block command actions to the terminal.

The primary use case would be passing the context of a terminal block directly to the ai chat mirroring the functionality of the Ask AI functionality in the ai-editor:

• the user clicks on a terminal command block and select the 'Ask AI' tool tip
• after entering a prompt into the window the user, the prompt and terminal context is passed into the ai chat

Currently the terminal variable is passed in as plain text and it would be nice to hide this behind the abstraction of an AI variable. The selection of the terminal command directly in the chat input window is just a nice to have.

The toolcall usecase is also good point, I can create an issue for this.

[eneufeld]

Looks good to me overall and works great.
I left some comment. The missing disposal is the main missing one.

[Fangoling]

I also adjusted the 'prompt' on how the command and output are passed to the terminal. They are now passed in this shape:

### Terminal Command
{command}
### Terminal Output
{output}
```.
And added an warn log for the invalid input branch.