feat: implement MCP tasks and RFC 6154 mailbox roles

[WeekendSuperhero]

Summary

• Added MCP task management to support background execution and polling of long-running tools.
• Replaced hardcoded mailbox names with auto-detection using RFC 6154 special-use attributes (\Trash, \Drafts).
• Removed custom CompatStdioWorker and switched to the standard rmcp stdio transport.
• Serialized destructive tool executions per-account using async mutexes.

Changes

Mailbox Roles

• Removed explicit trash_mailbox and drafts_mailbox settings from AccountConfig.
• Added role_from_attributes to parse RFC 6154 roles with fallback logic for older servers.
• Updated MailboxInfo to expose no_select, no_inferiors, and role.

MCP Server & Tasks

• Implemented TaskManager handling task enqueue, listing, info, results, and cancellation.
• Added task_support = "optional" to all applicable tools.
• Enforced synchronization for destructive tools via destructive_locks.

Dependencies & Config

• Added parking_lot crate for task state synchronization.
• Removed the Outlook provider from MailProvider.

Test plan

• Verify tools run correctly as async tasks and correctly report completion status.
• Check that destructive tools on the same account execute sequentially.
• Ensure Trash and Drafts mailboxes are correctly identified on legacy and RFC 6154-compliant servers.
• Validate standard MCP client connections succeed via rmcp stdio transport.

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PR description generated by Agent