Gmail connector should support multiple named accounts

Feature request: allow the Gmail connector to connect and target multiple Google accounts at the same time.

Current behavior: the assistant appears to access only one active Gmail account. This makes email-agent workflows incomplete when a user has more than one inbox.

Expected behavior:

• Support multiple connected Gmail accounts.
• Give each account a stable alias.
• Let prompts and tool calls target a specific account or an explicit set of accounts.
• Show the source account in search results, summaries, task extraction, and action confirmations.
• Default replies and drafts to the original receiving account.
• Let automations monitor selected accounts without duplicate alerts.
• Fail closed when the requested account is ambiguous.

This is needed for reliable multi-inbox email-agent workflows.