mark mode: begin selection at focused search result

[carlos-zamora]

Summary of the Pull Request

Searching in terminal highlights all search results. However, those results are considered separate from a selection. In the past, the highlighted result would be selected, resulting in it being the initial position for mark mode. Now that it's separate, mark mode doesn't start there.

To fix this, there's 2 changes here:

1. When we exit the search, we now select the focused search result. This becomes the initial position for mark mode.
2. When we're in the middle of a search and mark mode becomes enabled, the focused search result becomes the initial position for mark mode.

With this change, mark mode's initial position is determined in this order:

1. the position of an active selection
2. the position of the focused search result (if one is available)
3. the top-left position of the viewport (if there is a scrollback) (see mark mode: begin selection at viewport when scrolled up #19549)
4. the current cursor position

Validation Steps Performed

Entering mark mode in scenario X results in a starting position of Y:
✅ selected text during a search --> selected text

• NOTE: this seems to only occur if you start a search, then manually click on the terminal to bring focus there, but keep the search results active

✅ performed a search and results are available -->focused search result
✅ performed a search and no results are available

• scrolled up --> top-left of viewport
• no scrollback --> cursor position

✅ performed a search, got results, then closed search --> focused search result

Closes #19358

[carlos-zamora]

Personally, I think leaving behind selected text after performing a search feels a little weird, but I think it's necessary. This enables the user to begin a selection at the focused search result using only the keyboard. If we get rid of this, the user would need to manually click on the terminal or transfer focus over somehow.

[DHowett]

Hmmmm. I don't love that we now have three search states: active result, inactive result, and selected. I'm not sure how I feel all-up