Add caching for Slack channels in SlackWebMessagingIntegration

- Introduced a channels cache to optimize channel ID retrieval.
- Implemented methods to load and refresh the channels cache.
- Updated _get_channel_id to utilize the cache, improving performance and reducing API calls.

Author: MikaKerman

elementary/messages/messaging_integrations/slack_web.py

@@ -44,6 +44,7 @@ def __init__(self, client: WebClient, tracking: Optional[Tracking] = None) -> No
         self.client = client
         self.tracking = tracking
         self._email_to_user_id_cache: Dict[str, str] = {}
+        self._channels_cache: Optional[Dict[str, str]] = None
 
     @classmethod
     def from_token(
@@ -143,11 +144,31 @@ def _iter_channels(self, cursor: Optional[str] = None) -> Iterator[dict]:
                 raise ValueError("Next cursor is not a string")
             yield from self._iter_channels(next_cursor)
 
+    def _load_channels(self) -> None:
+        self._channels_cache = {}
+        try:
+            for channel in self._iter_channels():
+                self._channels_cache[channel["name"]] = channel["id"]
+            logger.debug(f"Loaded {len(self._channels_cache)} channels into cache")
+        except Exception as e:
+            logger.error(f"Failed to load channels: {e}")
+            # Keep cache as empty dict rather than None to avoid retrying constantly
+            self._channels_cache = {}
+
+    def _ensure_channels_loaded(self) -> None:
+        if self._channels_cache is None:
+            self._load_channels()
+
+    def refresh_channels_cache(self) -> None:
+        self._load_channels()
+
     def _get_channel_id(self, channel_name: str) -> str:
-        for channel in self._iter_channels():
-            if channel["name"] == channel_name:
-                return channel["id"]
-        raise MessagingIntegrationError(f"Channel {channel_name} not found")
+        self._ensure_channels_loaded()
+
+        if channel_name not in self._channels_cache:
+            raise MessagingIntegrationError(f"Channel {channel_name} not found")
+
+        return self._channels_cache[channel_name]
 
     def _join_channel(self, channel_id: str) -> None:
         try: