feat: Evaluation Client — Lifecycle, Orchestration & Online Pipeline

## Problem

The SDK's `EvaluationClient` only exposes `run()`. On the control plane side, customers cannot programmatically create custom evaluators (LLM-as-a-judge configs), list available evaluators, update or delete evaluators, or manage online evaluation configs for continuous evaluation on live traffic — evaluator provisioning requires the console. On the data plane side, the starter toolkit's `EvaluationProcessor` provides significantly richer orchestration than `run()`: it fetches session data from CloudWatch independently, groups evaluators by level (SESSION vs TRACE), determines which spans to send based on evaluator level, and runs multiple evaluators with per-evaluator error handling. The toolkit also provides input validation, IAM role cleanup on delete, and typed config/result models.

## Acceptance Criteria

- [ ] Customers can create, get, list, update, and delete custom evaluators
- [ ] Customers can create, get, list, update, and delete online evaluation configs
- [ ] Online evaluation config supports enable/disable toggling and sampling rate adjustment
- [ ] Typed result models with error introspection (`has_error()`, `get_successful_results()`)
- [ ] Customers can fetch session trace data (spans + runtime logs) from CloudWatch for a given session and agent
- [ ] Customers can find the most recent session for an agent
- [ ] Multi-evaluator orchestration groups evaluators by level and selects appropriate spans per level
- [ ] Per-evaluator error handling — failures on one evaluator don't block others
- [ ] Online evaluation config deletion supports optional IAM execution role cleanup
- [ ] All functionality is verified via integration tests running in CI

## Relevant Links

- [`EvaluationControlPlaneClient`](https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/4b9387f0d48cb6639633437b669fc6cc09ef07be/src/bedrock_agentcore_starter_toolkit/operations/evaluation/control_plane_client.py#L21)
- [`create_evaluator()`](https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/4b9387f0d48cb6639633437b669fc6cc09ef07be/src/bedrock_agentcore_starter_toolkit/operations/evaluation/control_plane_client.py#L107)
- [`create_online_evaluation_config()`](https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/4b9387f0d48cb6639633437b669fc6cc09ef07be/src/bedrock_agentcore_starter_toolkit/operations/evaluation/control_plane_client.py#L175)
- [`update_online_evaluation_config()`](https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/4b9387f0d48cb6639633437b669fc6cc09ef07be/src/bedrock_agentcore_starter_toolkit/operations/evaluation/control_plane_client.py#L312)
- [`EvaluationResult` / `EvaluationResults`](https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/4b9387f0d48cb6639633437b669fc6cc09ef07be/src/bedrock_agentcore_starter_toolkit/operations/evaluation/models.py#L81)
- [`OnlineEvaluationConfig`](https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/4b9387f0d48cb6639633437b669fc6cc09ef07be/src/bedrock_agentcore_starter_toolkit/operations/evaluation/models.py#L190)
- [`EvaluationProcessor`](https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/4b9387f0d48cb6639633437b669fc6cc09ef07be/src/bedrock_agentcore_starter_toolkit/operations/evaluation/on_demand_processor.py#L27)
- [`evaluate_session()`](https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/4b9387f0d48cb6639633437b669fc6cc09ef07be/src/bedrock_agentcore_starter_toolkit/operations/evaluation/on_demand_processor.py#L418)
- [`fetch_session_data()`](https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/4b9387f0d48cb6639633437b669fc6cc09ef07be/src/bedrock_agentcore_starter_toolkit/operations/evaluation/on_demand_processor.py#L88)
- [`determine_spans_for_evaluator()`](https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/4b9387f0d48cb6639633437b669fc6cc09ef07be/src/bedrock_agentcore_starter_toolkit/operations/evaluation/on_demand_processor.py#L304)
- [`execute_evaluators()`](https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/4b9387f0d48cb6639633437b669fc6cc09ef07be/src/bedrock_agentcore_starter_toolkit/operations/evaluation/on_demand_processor.py#L343)
- [`EvaluationDataPlaneClient`](https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/4b9387f0d48cb6639633437b669fc6cc09ef07be/src/bedrock_agentcore_starter_toolkit/operations/evaluation/data_plane_client.py#L20)
- [`delete_online_evaluation_config()`](https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/4b9387f0d48cb6639633437b669fc6cc09ef07be/src/bedrock_agentcore_starter_toolkit/operations/evaluation/online_processor.py#L200)

Bắt đầu với operations/evaluation/control_plane_client.py và models.py, sau đó đọc EvaluationProcessor trong on_demand_processor.py, EvaluationDataPlaneClient và delete_online_evaluation_config() trong online_processor.py. Trước tiên, hãy lần theo các entry point được liên kết và thiết lập integration test hiện có. Được xem là hoàn thành khi tất cả các tiêu chí được liệt kê về evaluator, cấu hình online, dữ liệu phiên, orchestration, kết quả lỗi, cleanup và integration test của CI đều được bao phủ.

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