Fix error resposes for `from` search prefix (#17963)

* Fix error responses in `from` search prefix (addresses mastodon/mastodon#17941)

Using unsupported prefixes now reports a 422; searching for posts from an
account the instance is not aware of reports a 404. TODO: The UI for this
on the front end is abysmal.

Searching `from:username@domain` now succeeds when `domain` is the local
domain; searching `from:@username(@domain)?` now works as expected.

* Remove unused methods on new Error classes as they are not being used

Currently when `raise`d there are error messages being supplied, but
this is not actually being used. The associated `raise`s have been
edited accordingly.

* Remove needless comments

* Satisfy rubocop

* Try fixing tests being unable to find AccountFindingConcern methods

* Satisfy rubocop

* Simplify `from` prefix logic

This incorporates @ClearlyClaire's suggestion (see
https://github.com/mastodon/mastodon/pull/17963#pullrequestreview-933986737).

Accepctable account strings in `from:` clauses are more lenient than
before this commit; for example, `from:@user@example.org@asnteo +cat`
will not error, and return posts by @user@example.org containing the
word "cat". This is more consistent with how Mastodon matches mentions
in statuses. In addition, `from` clauses will not be checked for
syntatically invalid usernames or domain names, simply 404ing when
`Account.find_remote!` raises ActiveRecord::NotFound.

New code for this PR that is no longer used has been removed.

app/controllers/api/v2/search_controller.rb

@@ -11,6 +11,10 @@ class Api::V2::SearchController < Api::BaseController
   def index
     @search = Search.new(search_results)
     render json: @search, serializer: REST::SearchSerializer
+  rescue Mastodon::SyntaxError
+    unprocessable_entity
+  rescue ActiveRecord::RecordNotFound
+    not_found
   end
 
   private

app/lib/search_query_transformer.rb

@@ -88,14 +88,14 @@ class SearchQueryTransformer < Parslet::Transform
       case prefix
       when 'from'
         @filter = :account_id
-        username, domain = term.split('@')
-        account = Account.find_remote(username, domain)
 
-        raise "Account not found: #{term}" unless account
+        username, domain = term.gsub(/\A@/, '').split('@')
+        domain           = nil if TagManager.instance.local_domain?(domain)
+        account          = Account.find_remote!(username, domain)
 
         @term = account.id
       else
-        raise "Unknown prefix: #{prefix}"
+        raise Mastodon::SyntaxError
       end
     end
   end

lib/exceptions.rb

@@ -10,6 +10,7 @@ module Mastodon
   class StreamValidationError < ValidationError; end
   class RaceConditionError < Error; end
   class RateLimitExceededError < Error; end
+  class SyntaxError < Error; end
 
   class UnexpectedResponseError < Error
     attr_reader :response