[FIX] mail: no crash when mentioning partner after channel mention

Steps to reproduce:
- Open channel general
- type "#" in composer so it prompts "#general"
- delete "#"
- type "@" in composer

This happens because the partner suggestion part relies on raw server
data in model thread, `thread.channel`, which is erased from the
channel suggestion. This was used to detect whether partner is member
or not, in `thread.channel.channelMembers`.

This commit fixes the issue by making partner suggestion relies on
more reliable data in models, `thread.channelMembers`.
`thread.channel` is deprecated, and will eventually be removed.

closes odoo/odoo#119935

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/mail/static/src/composer/suggestion_service.js

@@ -216,9 +216,9 @@ export class SuggestionService {
             if (!isAInternalUser && isBInternalUser) {
                 return 1;
             }
-            if (thread?.channel) {
-                const isMember1 = thread.channel.channelMembers[0][1].includes(p1);
-                const isMember2 = thread.channel.channelMembers[0][1].includes(p2);
+            if (thread.model === "discuss.channel") {
+                const isMember1 = thread.channelMembers.some((member) => member.persona === p1);
+                const isMember2 = thread.channelMembers.some((member) => member.persona === p2);
                 if (isMember1 && !isMember2) {
                     return -1;
                 }