Gateway hangs indefinitely — no response, no timeout error (Lambda-backed target, streaming disabled)

### Description

AgentCore Gateway accepted an HTTP POST (tools/call) but never returned any HTTP response — no 200, no error, no timeout. The calling client has been hung for 50+ minutes waiting for a response. Response streaming is disabled (default). The gateway target is Lambda-backed (no container cold start involved). Other requests to the same gateway in the same time window completed successfully.

### Steps to Reproduce

1. Agent runtime sends concurrent tool calls through ACG Gateway (Lambda-backed target)
2. Gateway accepts the HTTP POST request
3. Gateway never returns any HTTP response
4. Client httpx connection hangs indefinitely — no timeout error surfaced

This is intermittent and appears under concurrent load (3+ executions sharing the same agent container, all calling the same gateway simultaneously).

### Expected Behavior

Gateway should return an HTTP response within its configured timeout (40s based on our observations), either with the tool result or a timeout error.

### Actual Behavior

Gateway accepted the request and never responded. No HTTP response at all — not a timeout error, not a 5xx, nothing. The execution has been stuck for 50+ minutes with the httpx client waiting on an open connection.

### CLI Version

N/A — this is a Gateway service-side issue, not CLI.

### Operating System

Linux

### Additional Context

- Account: 272994241044
- Region: us-west-2
- Gateway: releaseautopilot-gateway-prod-internal-p03wrf71h7 (Lambda-backed)
- Timestamp: 2026-05-27 01:55:03 UTC (request sent, never received response)
- Response streaming: disabled (not configured, using default)
- Evidence from CloudWatch logs: The agent container continued processing other executions successfully after this request hung. The last log line for the affected trace (6a164e821d6483d72bc4ada63789a8d9) is the outbound HTTP POST to the gateway URL — then zero activity for 50+ minutes.
- Related: We also observe a pattern where the same gateway returns "Timed out waiting for tool execution after 900 seconds" errors after exactly ~40s wall-clock time (100 occurrences over 3 days). This hung case appears to be the same delivery failure but without the timeout error being surfaced.