fix(discord): resolve slash commands from interaction payload instead of gateway cache

Context.Guild in NetCord resolves the Guild object from the gateway client cache
(cache.Guilds.GetValueOrDefault(guildId)), not from the interaction JSON payload.
After a bot restart, the guild may not yet be cached when the first slash command
arrives, causing Context.Guild to be null even though the command is invoked
inside a guild channel. This produced "This command can only be used in a guild."

Changes:
- DiscordListSessionsCommand: use Context.Interaction.GuildId instead of Context.Guild.Id
- DiscordNewSessionCommand: use Context.Interaction.GuildId + REST GetGuildAsync/GetGuildUserAsync
- DiscordRescheduleCommand: same as above
- DiscordSessionInteractionModule: same fix for button interactions (CreateInput)
- Add null guard in GetResolvedPermissions for safety
- Bump version to 3.0.5

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/GmRelay.DiscordBot/Features/Sessions/DiscordSessionInteractionModule.cs

@@ -168,7 +168,7 @@ public sealed class DiscordSessionInteractionModule(
 
     private DiscordSessionInteractionInput CreateInput(Guid sessionId)
     {
-        var guild = Context.Guild
+        var guildId = Context.Interaction.GuildId?.ToString(CultureInfo.InvariantCulture)
             ?? throw new InvalidOperationException("Session buttons can only be used in a guild.");
         var message = Context.Interaction.Message
             ?? throw new InvalidOperationException("Session button interaction must include a message.");
@@ -176,7 +176,7 @@ public sealed class DiscordSessionInteractionModule(
         return new DiscordSessionInteractionInput(
             SessionId: sessionId,
             InteractionId: Context.Interaction.Id.ToString(System.Globalization.CultureInfo.InvariantCulture),
-            GuildId: guild.Id.ToString(CultureInfo.InvariantCulture),
+            GuildId: guildId,
             ChannelId: Context.Channel.Id.ToString(CultureInfo.InvariantCulture),
             MessageId: message.Id.ToString(CultureInfo.InvariantCulture),
             UserId: Context.User.Id,