Support Gmail delegation with runtime delegateFor parameter

[pbranchu]

Problem

All Gmail tools hardcode userId: 'me' when calling the Gmail API. This prevents using Gmail delegation, where one authenticated account (e.g., a service/assistant account) accesses another user's mailbox as a delegate.

Google's Gmail API supports passing the delegate's email address as userId (see Gmail API documentation). When delegation is set up between accounts, you can access another user's mailbox by passing their email as userId.

Fix

Adds an optional delegateFor parameter to all 28 Gmail tools. When provided, it overrides userId to target the delegated mailbox:

userId: args.delegateFor || await getAccountEmail(args.account)

This allows a single authenticated account to access multiple delegated mailboxes at runtime without needing separate account configurations for each.

Example usage

{
  "tool": "listGmailMessages",
  "args": {
    "account": "jeeves",
    "delegateFor": "philippe@example.com",
    "query": "is:unread"
  }
}

Backward Compatibility

Fully backward compatible:

• Without delegateFor: Falls back to getAccountEmail(args.account), which returns the account's own email. Gmail treats this identically to 'me'.
• With delegateFor: Passes the specified email as userId, using Gmail's built-in delegation support.

Testing

• Verified the project compiles cleanly with tsc --noEmit.
• No runtime behavior change for non-delegation users.
• Tested with a Workspace service account acting as delegate for multiple users.