Tasks: Log snapshot storage endpoint #1253

Repo: coder/coder

Scope: Add coderd agent endpoint for shutdown scripts to POST log snapshots.

Changes:

• Add POST /api/v2/workspaces/{workspace}/agents/me/tasks/{task}/snapshot?format=agentapi endpoint
• Request body: raw AgentAPI payload for format=agentapi (clients do not send the DB wrapper)
• Validate: task belongs to workspace, payload ≤ 64KB, decodes as AgentAPI payload schema
• Store in task_snapshots table (upsert, overwrite on each pause):
  • log_snapshot wrapper: {"format":"agentapi","data":<raw_payload>}
• Uses workspace agent token for auth

Files:

• coderd/aitasks.go (or new coderd/aitasks_snapshot.go)
• codersdk/aitasks.go
• coderd/coderd.go (route registration)

Acceptance criteria:

• Endpoint accepts valid snapshot and returns 204
• Validates format query parameter (currently only agentapi supported)
• Rejects payloads > 64KB with 400
• Rejects invalid task/workspace combinations with 404
• Overwrites existing snapshot for same task
• Uses agent token auth
• Stores snapshot in DB wrapper format: {"format":"agentapi","data":<raw_payload>}

Dependencies:

• Tasks: Database migrations for task lifecycle #1250
• Bug: Agent loses authorization before run_on_stop script completes coder#19467

References:

• PRD: Start/Pause/Resume the Task Workspace
• RFC: Tasks: Start/Pause/Resume Lifecycle: Log Snapshots, API Changes