feat(gmail): add listDrafts, getDraft, updateDraft, and deleteDraft operations

[AojdevStudio]

Summary

Gmail draft management is incomplete. We have createDraft and sendDraft but no way to list, read, update, or delete drafts programmatically.

Problem

When creating multiple draft iterations (e.g., rewriting an email), there's no way to:

• List existing drafts to find the right one
• Delete old/superseded drafts
• Update a draft in place instead of creating a new one
• Read a draft's content for review

Proposed Operations

Operation	Purpose	Gmail API
listDrafts	List all drafts (ID, subject, snippet)	users.drafts.list
getDraft	Get full draft content by ID	users.drafts.get
updateDraft	Update an existing draft's content	users.drafts.update
deleteDraft	Delete a draft by ID	users.drafts.delete

Context

Hit this limitation while drafting an email reply — needed to delete superseded drafts after rewriting, but listDrafts returned "Operation not found". Had to tell the user to manually clean up drafts in Gmail.

Acceptance Criteria

• listDrafts returns draft IDs, subjects, and snippets
• getDraft returns full draft content (to, subject, body, cc, bcc)
• updateDraft modifies an existing draft without creating a new one
• deleteDraft removes a draft by ID
• All four operations registered in the Gmail service spec

[linear]

GDRIVE-11