feat(discord): improve UX and add source-level tests for /newsession

- DiscordNewSessionCommand: on success, renders session details via
  DiscordSessionBatchRenderer.Render() with embeds and action rows.
- DiscordNewSessionCommand: uses Discord emoji shortcodes for error
  and success messages (✅, ⛔, 💥).
- DiscordNewSessionHandlerTests: added 7 source-level structural tests
  verifying Dapper usage, NpgsqlDataSource, permission checks,
  platform neutrality, transaction safety, CancellationToken usage,
  and embed rendering in the command.

Refs issue #28

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

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

@@ -1,3 +1,4 @@
+using GmRelay.DiscordBot.Rendering;
 using NetCord.Rest;
 using NetCord.Services.ApplicationCommands;
 
@@ -49,19 +50,23 @@ public class DiscordNewSessionCommand : ApplicationCommandModule<SlashCommandCon
                 joinLink: link,
                 CancellationToken.None);
 
+            var (embeds, actionRows) = DiscordSessionBatchRenderer.Render(view);
             await Context.Interaction.SendResponseAsync(
-                InteractionCallback.Message("+ Session created!"));
+                InteractionCallback.Message(new InteractionMessageProperties()
+                    .WithContent(":white_check_mark: **Session created successfully!**")
+                    .WithEmbeds(embeds)
+                    .WithComponents(actionRows)));
         }
         catch (UnauthorizedAccessException ex)
         {
             await Context.Interaction.SendResponseAsync(
-                InteractionCallback.Message($"! {ex.Message}"));
+                InteractionCallback.Message($":no_entry: {ex.Message}"));
         }
         catch (Exception ex)
         {
             _logger.LogError(ex, "Failed to create session for user {UserId} in guild {GuildId}", Context.User.Id, guild.Id);
             await Context.Interaction.SendResponseAsync(
-                InteractionCallback.Message("* An error occurred while creating the session."));
+                InteractionCallback.Message(":boom: An error occurred while creating the session."));
         }
     }