Add BotId and EnterpriseId to AuthTestResponse

[vgribok]

auth.test documents two response fields that AuthTestResponse doesn't carry:
bot_id and enterprise_id.

• bot_id is returned whenever the call is made with a bot token. It identifies
  the app installation and is distinct from user_id, so it can't be derived
  from the fields already present.
• enterprise_id is returned for a token belonging to a team within an
  Enterprise Grid org.

Reference: https://docs.slack.dev/reference/methods/auth.test

Why it's useful

Slack ties chat.delete authority and DM-history visibility to the app
installation rather than the bot user. When a delete fails with
cant_delete_message, comparing auth.test's bot_id against the message's own
bot_id is what distinguishes "a different installation posted this" from a real
failure. MessageEvent already exposes BotId, so today only one half of that
comparison is reachable.

Notes

• No JsonProperty attributes needed — the snake_case naming strategy in
  SlackJsonSettings maps both. Verified by deserializing a documented
  bot-token auth.test body.
• Both fields are conditionally present, so they're string? under
  #nullable enable, following OauthV2AccessResponse.

Verification

SlackNet.Tests passes: 585 passed, 0 failed.