gmail threads get --format full drops MIME payload from JSON

himalaya --json gmail threads get <id> --format full fetches the full Gmail MIME payload but drops it when constructing GmailThreadMessageOutput. The JSON contains headers, labels and snippets, but no bodies or attachment metadata. Consumers cannot read a full conversation or discover attachment IDs from this command.

In src/gmail/threads/get.rs, retain message.payload as an optional serialized GmailMessagePayload field, using a clone for message_headers. Keep the existing text display unchanged. This also exposes the payload in the generated gmail-threads-get.json schema.

A working downstream patch is himalaya-thread-payload.patch in olai PR #610. It builds against our 2.1.0 pin; olai validates the generated schema and decodes nested plain text, HTML and attachment metadata. The same payload-dropping code is still present on Himalaya master.

Expected: full-format JSON includes each message's MIME payload. Metadata and minimal responses may omit the field when Gmail supplies none. Once this lands upstream, olai can remove its patch on the next pin bump.