feat(google-mail): Add threading support (thread_id, In-Reply-To, References, get-email)

[frankhli843]

Problem

Emails sent via the Gmail MCP tool (send-email) cannot be threaded correctly for recipients. The threadId parameter only groups messages on the sender's Gmail — recipients see each reply as a separate conversation.

This is because:

1. send-email has no parameters for thread_id, in_reply_to, or references headers
2. There is no get-email action to retrieve the Message-ID header from a sent/received message
3. Without knowing the Gmail-assigned Message-ID, it's impossible to set proper In-Reply-To and References headers on replies

Background

Per RFC 2822 and Gmail's threading logic, recipient-side threading requires:

• In-Reply-To: <Message-ID of parent email>
• References: <space-separated Message-IDs of conversation chain>
• Subject: Re: <original subject>

The threadId in the Gmail API request body only affects the sender's mailbox. Recipients rely entirely on these RFC headers.

Proposed Changes

1. Add threading parameters to send-email and create-draft

• thread_id (string, optional) — Gmail thread ID for sender-side threading
• in_reply_to (string, optional) — RFC 2822 Message-ID of the email being replied to
• references (string[], optional) — List of Message-IDs from the conversation chain

2. Add new get-email action

• message_id (string, required) — The ID of the message to retrieve
• format (enum, optional) — full, metadata, minimal, raw
• metadata_headers (string[], optional) — Specific headers to return (e.g., ["Message-ID", "In-Reply-To", "References"])

3. Backend: pass threading headers through to Gmail API

The invoke-action backend needs to:

• Include threadId in the Gmail API send request body
• Set In-Reply-To and References in the raw MIME when provided
• Return Message-ID header in get-email responses

Intended Workflow

1. Receive/find an email → note its Gmail id
2. Call get-email with format=metadata, metadata_headers=["Message-ID"] → get the RFC Message-ID
3. Call send-email with thread_id, in_reply_to=<that Message-ID>, references=[<that Message-ID>], subject="Re: ..."
4. Email threads correctly for both sender AND recipient ✅

Reference Implementation

I have a working fork with the client-side changes (parameters, prompts, tools, configuration):
https://github.com/frankhli843/agent-toolkit/tree/fix/gmail-threading-support

Happy to open a PR if helpful.