Add assistantmail_reply_message tool

[dmcrider]

The existing assistantmail_send_email tool calls POST /v1/mailboxes/{mailboxId}/messages. The new tool follows the same pattern but hits the reply endpoint:

POST /v1/mailboxes/{mailboxId}/messages/{messageId}/reply
Body: { text?: string, html?: string } 

The tool definition needs:

• Input params: mailboxId, messageId (the message being replied to), text, html
• No to, subject, or threading headers — the backend derives all of those automatically from the original message
• Returns { messageId, status, inReplyTo, subject } (the auto-generated reply subject is echoed back)