feat(slack): expose user email in message.author

[Seventy7dot2]

Summary

Resolves #237

Adds support for exposing Slack user email in message.author.email
when the users:read.email scope is granted.

Test plan

• Created a Slack app with the following OAuth scopes:
  • users:read
  • users:read.email
• Installed the app in a test Slack workspace.
• Triggered message events (app_mention and message) to ensure user resolution is performed by the SDK.
• Verified that the user's email is correctly populated in message.author.email.
• Confirmed that when the users:read.email scope is not granted, the email field remains undefined and existing behavior is unchanged.
• Ensured no breaking changes to the existing user object structure.

[vercel]

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