feat(mail): Add support for getting Gmail threads

[krisrowe]

Summary

Add the capability to retrieve a full Gmail thread, including all its messages, via both the gwsa CLI and the MCP server.

This addresses the need to view the entire context of a conversation, which is not possible with the current gwsa mail read command that only fetches a single message.

Context: Distinguishing Message vs. Thread IDs

It is critical that the implemented solution clearly addresses the distinction between Gmail message_id and thread_id at a glance, both in output and API design. MCP clients (like the Gemini CLI) need to reliably identify which type of ID they are being given when asked about an email. If necessary, build error messaging into the MCP tools or make them versatile enough to handle either type of ID gracefully.

Acceptance Criteria

1. SDK Layer (gwsa/sdk/mail/read.py):

   • A new function get_thread(thread_id: str) should be created.
   • This function must call the service.users().threads().get() method of the Gmail API.
   • The function should process the raw API response to return a simplified list of messages within the thread, including essential headers like id, subject, from, to, date, and snippet for each message.
   • The from key in the message dictionary must be correctly handled as it's a Python reserved keyword (e.g., by quoting it: {"from": ...}).

2. CLI Layer (gwsa command):

   • A new CLI command gwsa mail threads get <thread_id> should be created.
   • This command should be implemented in a new file: gwsa/cli/mail/threads.py.
   • It must call the gwsa.sdk.mail.read.get_thread() function.
   • The output should be the full thread object, formatted as a pretty-printed JSON string.
   • The new threads subcommand must be correctly wired into the main mail command group in gwsa/cli/__main__.py.

3. MCP Tool Layer (gwsa-mcp):

   • A new MCP tool named get_email_thread(thread_id: str) should be added to gwsa/mcp/server.py.
   • The tool should have a clear docstring explaining its purpose and arguments.
   • It must call the gwsa.sdk.mail.read.get_thread() function and return the resulting dictionary.

Implementation Notes

• Logic should be centralized in the SDK layer.
• Follow existing patterns for SDK functions, CLI commands (click), and MCP tool definitions.
• Ensure appropriate error handling is in place for all layers.

Gmail API Reference

• Threads: get method documentation