HTTP runtime gateway targets cannot configure schema for policy guardrails

### Description

AgentCore Gateway HTTP runtime targets support an optional API schema. However, AWS documentation states that HTTP runtime agents _are required_ provide this schema when using guardrails.

The CLI exposes --schema for gateway targets, but currently rejects it for `--type http-runtime`. Consequently, users cannot configure policy guardrails correctly for HTTP runtime targets.

### Steps to Reproduce

Reproduce `--schema` validation rejection
1. create HTTP runtime with a policy engine
2. Add an HTTP runtime target while passing `--schema` flag

Reproduce deploy failure
1. create HTTP runtime with a policy engine
2. Add an HTTP runtime target without passing `--schema` flag
3. Deploy the target and apply guardrail policies
4. Attempt to invoke the target

### Expected Behavior

* The CLI should accept a schema for HTTP runtime targets
* benign requests permitted by policy should succeed

### Actual Behavior

* CLI rejects `--schema` for HTTP runtime targets
* benign requests permitted by policy are rejected due to missing schema

### CLI Version

_No response_

### Operating System

macOS

### Additional Context

* Accept and validate `--schema` for http-runtime in CLI
* persist schema source in agentcore.json
* persist schema through deploy