import-agent: Specify Bedrock AgentCore Gateway Targets

**Is your feature request related to a problem? Please describe.**

I'm attempting to use the **import-agent** tool to migrate a Bedrock Agent into AgentCore but I'm running into the following issue:

When I try to import a Bedrock Agent that has OpenAPI action group schemas, the import fails with validation errors. It looks like the tool is converting these to Gateway Lambda targets, which only support a limited schema format (type, properties, required, items, description). I can see that AgentCore Gateway supports OpenAPI targets with full schemas, so is this just a current limitation of the tool?

**Describe the solution you'd like**

The ability to specify an AgentCore Gateway target type, such as OpenAI schema, when configuring the migration of an agent using **import-agent**

**Describe alternatives you've considered**
The only alternative I can see is manually migrating the agent.

**Additional context**
Add any other context or screenshots about the feature request here.

import-agent のエントリポイントから開始し、OpenAPI アクショングループスキーマが Gateway Lambda ターゲットに変換される流れを追跡します。その動作を AgentCore Gateway の OpenAPI ターゲットと比較し、設定可能なターゲットタイプをどのように表現すべきかを判断します。現在のバリデーションエラーなしにインポートで OpenAPI 互換のターゲットを選択できれば完了です。

索引モデルが issue の本文から書いたものです。