Improve /add completion popup candidate display on long file names

[Dima-369]

Issue

I have a Kotlin Compose project and all source files are always displayed like this on /add which is cumbersome to use.

This means, I always have to arrow through the candidates to find the matching one. Works, but not too great.

Roo Code, as a comparison, displays the file names like this (just the end) which is often far more useful:

Version and model info

Aider v0.78.0
Model: openrouter/deepseek/deepseek-chat:free with diff edit format, prompt cache, infinite output
Git repo: .git with 272 files
Repo-map: using 4096 tokens, auto refresh
Added memory-bank/techContext.md to the chat.

[Dima-369]

Turns out that it is not that easy since that functionality is from the prompt_toolkit library. Locally, if I patch its menus.py with this function:

def _trim_formatted_text(
    formatted_text: StyleAndTextTuples, max_width: int
) -> tuple[StyleAndTextTuples, int]:
    """
    Trim the text to `max_width`, prepend dots when the text is too long.
    Returns (text, width) tuple.
    """
    width = fragment_list_width(formatted_text)

    # When the text is too wide, trim it.
    if width > max_width:
        result = []  # Text fragments.
        remaining_width = max_width - 3  # Reserve space for "..."
        fragments = list(explode_text_fragments(formatted_text))
        
        # Start from the end and work backwards
        for style_and_ch in reversed(fragments):
            ch_width = get_cwidth(style_and_ch[1])
            
            if ch_width <= remaining_width:
                result.insert(0, style_and_ch)  # Insert at start to maintain order
                remaining_width -= ch_width
            else:
                break
                
        result.insert(0, ("", "..."))  # Prepend the dots
        
        return result, max_width - remaining_width
    else:
        return formatted_text, width

I get this display, which is exactly what I like. One can also patch column width to be a bit nicer when the paths are so long (maybe maxing out at 2 columns), but this is good for the start.