Conduit: Validate "attachments" keys for correctness

Summary:
`PhabricatorApplicationSearchEngine::buildConduitResponse()` already does some validation for passed `constraints`.
It should do the same for passed `attachments`.

Closes T16525

Test Plan:
* Look at http://phorge.localhost/conduit/method/maniphest.search/#attachments and understand which attachment keys are valid for this endpoint.
* In the `attachments` field, enter nonsense like `{"subscribers":["foo","bar"],"invalidkey":["foo"]}` or `{"foo":"bar"}`, and press "Call Method"
* Get an error message after applying this patch
* In the `attachments` field, enter half-nonsense (at least valid keys) like `{"subscribers":["foo","bar"],"projects":[""]}`, and press "Call Method"
* Get results and no error, as before (as this patch only introduces validation of keys but not of values)

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16525

Differential Revision: https://we.phorge.it/D26796

src/applications/search/engine/PhabricatorApplicationSearchEngine.php

···
           'Parameter "attachments" must be a map of attachments, got "%s".',
           phutil_describe_type($attachment_specs)));
     }
+    foreach ($attachment_specs as $key => $attachment) {
+      if (empty($attachments[$key])) {
+        throw new Exception(
+          pht(
+            'Attachment key "%s" is not a valid attachment for this query.',
+            $key));
+      }
+    }
 
     $attachments = array_select_keys(
       $attachments,