feat(gateway-target): expose MetadataConfiguration for header / query parameter propagation

### Description

`AWS::BedrockAgentCore::GatewayTarget.MetadataConfiguration` exposes three properties for HTTP header / query-parameter propagation that the CLI does not surface:

- `AllowedRequestHeaders` — Array of String — headers propagated from MCP client → upstream target.
- `AllowedResponseHeaders` — Array of String — headers propagated from upstream → MCP client.
- `AllowedQueryParameters` — Array of String — query params propagated end-to-end.

Source: [MetadataConfiguration CFN docs](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-bedrockagentcore-gatewaytarget-metadataconfiguration.md).

CFN splits headers into request / response (unlike the data-plane API which uses a single `allowedHeaders`). The CLI's `AgentCoreGatewayTargetSchema` (`src/schema/schemas/mcp.ts:339–546`) has no `metadataConfiguration` block, and the L3 construct (`@aws/agentcore-cdk` — `src/cdk/constructs/components/mcp/Gateway.ts`) does not pass anything through.

This is the GatewayTarget counterpart to the existing `requestHeaderAllowlist` plumbing on Runtime (`agent-env.ts:123–163`) — same shape can be reused.

### Acceptance Criteria

- [ ] Add a `metadataConfiguration` block to `AgentCoreGatewayTargetSchema`:
- `allowedRequestHeaders?: string[]`
- `allowedResponseHeaders?: string[]`
- `allowedQueryParameters?: string[]`
- [ ] Add CLI flags: `--allowed-request-headers`, `--allowed-response-headers`, `--allowed-query-parameters` (each repeatable or comma-separated).
- [ ] Add corresponding TUI fields in `AddGatewayTargetFlow.tsx`.
- [ ] Pass through to `CfnGatewayTarget.MetadataConfiguration.{AllowedRequestHeaders, AllowedResponseHeaders, AllowedQueryParameters}` in the L3 construct.
- [ ] Validate against AgentCore-reserved prefixes (`x-amz-*`, `x-amzn-*`) consistent with Runtime allowlist validation.
- [ ] Snapshot tests updated.

### Additional Context

- CFN support: **YES** — `GatewayTarget.MetadataConfiguration` (all three fields).
- L3 gap: yes — `@aws/agentcore-cdk` GatewayTarget construct needs corresponding plumbing.
- Naming nuance: CFN splits `Request` / `Response` headers; users will likely ask for a single "allowed headers" surface — keep both for parity with CFN, but document the distinction.
- Reuse the validator from `agent-env.ts:123–163` for header name format.