Add 'None' auth option for Gateway targets (OpenAPI) to support pass-through JWT

## Description

When creating a Gateway target from an OpenAPI spec file, the CLI only offers two outbound auth options: **API key** and **OAuth**. There is no **None** option.

This blocks a valid use case: customers who use an interceptor Lambda to pass through an existing JWT token. Their users are already authenticated, so no gateway-managed credential is needed — downstream services verify the JWT that gets passed through. Today these customers are forced to pick API key or OAuth even though neither will be used.

**Request:** Add a "None" (no outbound auth) option when creating an OpenAPI Gateway target.

## Acceptance Criteria

- [ ] `agentcore add gateway-target` with an OpenAPI spec offers a "None" auth option alongside API key and OAuth
- [ ] Selecting "None" configures the target with no outbound credential provider
- [ ] The resulting schema / CDK output correctly represents a target with no auth
- [ ] Documentation/help text explains the pass-through JWT use case for "None"

## Additional Context

Reported via customer feedback (Aperture form submission).

- CLI version: `0.21.0`
- OS: `linux 6.1.170-210.320.amzn2023.x86_64`, node `v20.19.3`

> When we create Gateway target to OpenAPI spec file. We have two auth options API key and OAuth. But there is no None option for cases when we want to use interceptor lambda to pass through existing JWT token, because users are already authenticated. But must choose one of these options and we will not use any of them. Downstream services will verify JWT we pass through.

Start at the `agentcore add gateway-target` command and trace the OpenAPI target authentication choices into the target schema and CDK output. Verify the None option appears alongside API key and OAuth, produces no outbound credential provider, and that the help or documentation explains the pass-through JWT use case.

Written by the indexing model from the issue text.