search: Respect spaces in person name in new suggestions only.

[shubham-padia]

Fixes #6515.
New suggestions for sender:King ha will respect spaces and the new
suggestion will be Sent by King Hamlet <email> instead of Sent by King, search for ha. But if first term of sender operand is a valid user email,
tokens will be seperated by spaces. e.g sender:hamlet@zulip.com abc
will show Sent by King Hamlet <email>, search for abc.

The original issue mentioned

considering things with spaces in them if and only if the thing after the : is an exact prefix of someone's name.

I think that will not be necessary in this case as we are using only emails for filtering. If it's not an exact prefix, e.g sender:Kin ha, the prefix check would enable to make the suggestion Sent by Kin, search for ha. But since Kin is not a valid email, it's of no use and any query following it would result in empty search. The result without the prefix check would be Sent by Kin Ha. Thus, I think it's better to skip this check and (presumably) save some time. (Let me know if this needs to be changed)

[zulipbot]

Hello @zulip/server-search members, this pull request was labeled with the "area: search" label, so you may want to check it out!

[timabbott]

Merged, thanks @shubham-padia! I think there's a broader range of low-priority similar issues that we should presumably also fix, like completing stream and topic names with a space in them.

But we might want to think about that as a broader rearchitecting of this system to have the pills structure (I think fundamentally, starting with Filter.parse won't necessarily be the right model there).

(I also think that the "Sent by king ham" top option is rarely what someone wants, and we should probably not be suggesting it as the top option)