aichat: allow user to select partial text from assistant's response

[nullhook]

after adding the ability to long-press assistant responses on mobile, it also disables the selection of partial text as a side effect. to accommodate text selection, we could introduce a 'select text' option in the context menu, potentially opening a modal where users can select text freely.

[mattmcalister]

@nullhook is this a duplicate of https://github.com/brave/brave-browser/issues/35640? or are you describing something else?

[nullhook]

this is something else