Feature Request: Session Export and Replay for Agent Debugging

**Title**

Feature Request: Session Export and Replay for Agent Debugging

## Description

### Problem

When building agents with the Copilot SDK, debugging complex multi-turn conversations can be difficult.

Developers often encounter situations where:

* An agent behaves unexpectedly after several turns.
* A tool invocation produces an incorrect result.
* Context accumulation affects later responses.
* Bugs only appear after a specific sequence of interactions.

Currently, reproducing these issues typically requires manually recreating the entire conversation flow.

### Proposed Solution

Add session export and replay capabilities to the Copilot SDK.

#### Session Export

Allow developers to export a session including:

* User messages
* Assistant messages
* Tool calls
* Tool results
* MCP interactions
* Session metadata
* Model configuration

Example:

```json
{
"sessionId": "...",
"messages": [...],
"toolCalls": [...],
"metadata": {...}
}
```

#### Session Replay

Provide APIs to replay exported sessions for debugging and testing.

Example:

```typescript
const session = await sdk.loadSession("./session.json");

await session.replay();
```

#### Optional Features

* Replay until a specific turn
* Step-by-step execution
* Compare outputs across SDK versions
* Compare outputs across models
* Deterministic replay mode

### Use Cases

#### Agent Development

Reproduce user-reported failures without requiring users to recreate conversations.

#### Regression Testing

Verify that SDK upgrades do not change agent behavior unexpectedly.

#### Tool Debugging

Inspect exactly how tool calls and results affected agent decisions.

#### Benchmarking

Compare responses across different models using the same session history.

### Benefits

* Easier debugging
* Better reproducibility
* Improved regression testing
* Faster issue triage
* Better developer experience for agent builders

### Alternatives Considered

* Manual logging of messages
* Custom session serialization
* Third-party observability platforms

These approaches require significant additional implementation and are not standardized across SDK users.

Bắt đầu bằng việc xem xét cách xử lý session hiện có của Copilot SDK và API loadSession/replay được đề xuất trong issue. Xác định cách biểu diễn các message đã export, các lần gọi tool, các tương tác MCP, metadata và cấu hình model, sau đó xác định tiêu chí hoàn tất cho việc export một session và replay session đó để debug hoặc kiểm thử.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.