Allow uppercase characters when creating tool key shortcuts

[kiennq]

This PR allows the use of uppercase characters when creating key shortcuts for tools.
This would be beneficial for tools like github mcp which provides more than 26 tools

[karthink]

Thanks for the PR @kiennq.

A couple of notes:

• The general convention I follow in gptel is that (with x = any lower case char, X = any upper case char)

  • -x sets a variable, i.e. some state that persists after the menu is closed,
  • xsets a transient menu infix option (i.e. not a suffix that quits the transient)
  • X or a special key (like RET) runs a suffix and quits the menu.
    Going by this convention, we should not use upper case letters for selecting tools.

  That said, I just broke this convention in the gptel-integrations menu items, so it's not a hard rule.

• However, a more serious problem is that the tools menu is almost unusable when you have a couple of MCP servers providing 25 tools each. Because it doesn't scale, we are thinking about replacing individual tool selection with completing-read-multiple in gptel-transient: tool selection using completing-read-multiple #653. Categories (i.e. all tools in the category) will still be selected with transient menu key bindings.

[kiennq]

• However, a more serious problem is that the tools menu is almost unusable when you have a couple of MCP servers providing 25 tools each. Because it doesn't scale, we are thinking about replacing individual tool selection with completing-read-multiple in gptel-transient: tool selection using completing-read-multiple #653. Categories (i.e. all tools in the category) will still be selected with transient menu key bindings.

I have the tool selection using completing-read-multiple enabled as well, however I still find myself time to time go to the full table view of all tools, especially for a quick glance and quickly enable a tool in the list.
Another thing is that the current table view provides a quick visual check of which tools have been enabled, and I think that's a good thing to keep.
Here is the commit that follows the completing-read-multiple that I used btw kiennq@e35cec0