Tasks: API: Add pagination and granular message control to logs endpoint

首先查看任务日志端点、coderd 现有的分页模式以及 codersdk.Pagination。确定活动日志和暂停快照的分页应如何工作，同时允许未来的 ACP 消息类型。完成的标准是：该方案已实现并记录文档，已针对两种状态进行测试，并且 CLI 已在需要时更新。

Problem

The task logs API (GET /api/v2/tasks/{user}/{task}/logs) currently returns the entire conversation history. This is not ideal for tasks with long-running conversations:

• Large response payloads for tasks with hundreds of messages
• No way to fetch specific message ranges
• Inefficient for clients that only need recent messages or want to implement infinite scroll

Current Behavior

The API returns all messages in a single response:

{
  "logs": [...],  // All messages
  "snapshot": false,
  "total_count": 250
}

When a task is paused (post-implementation of pause/resume functionality), it returns the last 10 messages from the snapshot:

{
  "logs": [...],  // Last 10 messages
  "snapshot": true,
  "snapshot_at": "2025-01-18T10:00:00Z",
  "total_count": 10
}

Proposed Enhancement

Add pagination support to the logs endpoint. The exact implementation will depend on the message model after ACP integration, which may introduce more granular message types, status updates, and metadata beyond the current large text blocks.

Pagination should follow coderd's existing patterns (using limit, offset, and after_id query parameters as defined in codersdk.Pagination).

Key considerations:

• With ACP, messages may be split into multiple granular events rather than 500-line text blocks
• The message structure and metadata may evolve significantly
• Pagination design should accommodate future message model changes

Context

Raised during RFC review for Task Start/Pause/Resume Lifecycle. Deferred from Beta/GA as it's an enhancement rather than blocking issue.

Acceptance Criteria

• Design pagination approach that accommodates future ACP message model changes
• Works for both active tasks (live logs) and paused tasks (snapshots)
• Follows coderd's existing pagination patterns
• Document pagination behavior
• Update CLI if needed to support fetching message ranges

Related

• RFC: Task Start/Pause/Resume Lifecycle
• Future: ACP integration for message-level control