[Bug] Gateway returns internal error for tools/call with OAuth MCP targets

## Summary

AgentCore Gateway fails to forward `tools/call` requests to MCP server targets configured with OAuth authentication (client_credentials flow). The Gateway returns "An internal error occurred" without forwarding the request to the MCP runtime.

## Which component is impacted?

- [ ] 01-AgentCore-runtime
- [x] 02-AgentCore-gateway
- [ ] 03-AgentCore-identity
- [ ] 04-AgentCore-memory
- [ ] 05-AgentCore-tools
- [ ] 06-AgentCore-observability
- [ ] 07-AgentCore-E2E

## What Works

- `tools/list` via Gateway (cached from synchronization) ✅
- Direct OAuth calls to MCP runtime (both `tools/list` and `tools/call`) ✅

## What Fails

- `tools/call` via Gateway → Returns "An internal error occurred" ❌

## Environment

- Region: us-east-1
- Gateway Authentication: AWS_IAM (SigV4)
- Target Authentication: OAuth credential provider (Cognito client_credentials)
- MCP Runtime: AgentCore Runtime with customJWTAuthorizer

## Reproduction Steps

1. Create MCP runtime with `customJWTAuthorizer` (Cognito OIDC discovery URL)
2. Create Gateway with AWS_IAM authentication
3. Create Gateway target with OAuth credential provider:
- Token endpoint: Cognito `/oauth2/token`
- Grant type: `client_credentials`
- Scope: custom scope for MCP invoke
4. Verify target status is READY (synchronization succeeds)
5. Call `tools/list` via Gateway → Works, returns tool list
6. Call `tools/call` via Gateway → Fails with internal error

## Error Response

```json
{"jsonrpc":"2.0","id":"1","error":{"code":-32603,"message":"An internal error occurred"}}
```

## Evidence

MCP runtime CloudWatch logs show only:
- `PingRequest` (during synchronization)
- `ListToolsRequest` (during synchronization)

**No `CallToolRequest` is ever received** by the runtime when calling through Gateway.

When calling the MCP runtime directly with OAuth Bearer token (bypassing Gateway), both `tools/list` and `tools/call` work correctly.

## Workaround

We implemented a workaround that calls the MCP runtime directly with OAuth tokens, bypassing the Gateway for `tools/call` requests. This confirms the issue is in the Gateway's request forwarding, not the MCP runtime or OAuth configuration.

## Expected Behavior

Gateway should forward `tools/call` requests to MCP runtime targets with OAuth authentication, just as it does for `tools/list` requests.

## AWS Support Case

Filed with AWS Support (Bedrock AgentCore / Gateway category).