[Server] Let users fetch their own private chat history (#7506)

* [Server] Let users fetch their own private chat history

Players lose their private conversations on reconnect or on another
device. Servatrice can already log private messages, but only
moderators can read the logs.

Add Command_GetPrivateChatHistory / Response_PrivateChatHistory so a
registered user can page through their own logged conversation with one
other user. Operators opt in with logging/allow_user_chat_history
(default off, and log_user_msg_chat must be on). The caller is matched
by account id so reused guest or deleted-account names can't read older
messages; only time, sender name and text are returned. Pages hold
1..200 messages and never split a second, since log_time has 1-second
resolution.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* [Server] Harden private chat history identity matching

Match both participants by account id and name so a federated user's remote id cannot collide with a local account, and stop storing foreign ids in the local chat log. Extract the paging assembly into a pure helper and cover the whole-second boundary rules with gtests.

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Lukas Brübach <44814898+BruebachL@users.noreply.github.com>

libcockatrice_protocol/libcockatrice/protocol/pb/session_commands.proto

@@ -37,6 +37,7 @@ message SessionCommand {
         DECK_SHARE_LIST_MINE = 1032;
         DECK_SHARE_REMOVE = 1033;
         SUBMIT_PASSWORD_VERIFIER = 1034;
+        GET_PRIVATE_CHAT_HISTORY = 1035;
         REPLAY_LIST = 1100;
         REPLAY_DOWNLOAD = 1101;
         REPLAY_MODIFY_MATCH = 1102;