feat(discord): Add Ephemeral Support

[DeanMauro]

Summary

Resolves #515. This PR adds support for ephemeral messages in Discord.

Because Discord's loading state (the initial "thinking..." response produced by DEFERRED_CHANNEL_MESSAGE_WITH_SOURCE) sets the visibility for the entire message and cannot be changed once set, the EPHEMERAL message flag must be set in the initial response (i.e. in this.respondToInteraction()). See here for more detail.

I've added a hook to the adapter that can conditionally set these flags, for example for particular slash commands.

    const discord = createDiscordAdapter({
      applicationId,
      botToken: env.DISCORD_BOT_TOKEN,
      publicKey: env.DISCORD_PUBLIC_KEY,
      mentionRoleIds: getMentionRoleIds(env.DISCORD_MENTION_ROLE_IDS),
      userName: env.BOT_USER_NAME,
      logger,
      flags: (context) => {
        if (context.command === '/ephemeral') {
          return DiscordMessageFlag.Ephemeral;
        }
      }
    });

Slash commands use the post() method regardless.

chat.onSlashCommand('/public', async event => {
   await event.channel.post("This is a public response. Anyone can see it.");
});

chat.onSlashCommand('/ephemeral', async event => {
   await event.channel.post("This is a private response. Only the user who issued the command can see it.");
});

The result looks like this:

Public
https://github.com/user-attachments/assets/039e13cc-279c-4440-b09a-3ecd3975fd81

Ephemeral
https://github.com/user-attachments/assets/8d5fde57-647d-4055-986a-d4866e9adbd7

Both the loading state and the final message are Ephemeral if the flag is set; public otherwise. Using postEphemeral() falls back to sending a private DM.

Notes

• I tried other methods like deleting the loading state message and creating a new one below it as a follow up and it just looks sloppy.
• Also considered a straight ephemeral: boolean field in the adapter config but that's too limiting for those who want per-command control.

Test plan

Created a public and private slash command and tested with a test app.

Checklist

• All commits are signed and verified
• pnpm validate passes
• Changeset added (or N/A — see CONTRIBUTING.md)
• Documentation updated (or N/A)

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