API Gateway emulator: HTTP proxy integration so non-Lambda backends share the emulator origin

[msmourao]

Describe the feature

The Lambda Test Tool v2 API Gateway emulator today models routes as Lambda integrations only (HTTP → APIGatewayProxyRequest / HTTP API v2 JSON → Runtime API).
Real API Gateway also supports HTTP proxy integrations. Locally, teams often need the same: some routes are Lambda, others are plain HTTP (Kestrel, containers, App Runner, etc.), but callers should keep one emulator base URL / origin.
Please add an optional route integration type on the emulator:

• Lambda (default) — current behavior.
• Http — reverse-proxy the incoming HTTP request to a configured Endpoint URL. Do not wrap the body as an API Gateway Lambda event. The client still talks to the emulator; the emulator talks to the HTTP process.

Use Case

Mixed local topologies where a frontend (or other clients) already call the emulator host for API Gateway. If a backend is not a Lambda, there is today no way for that route to stay on the same emulator domain, so callers must switch origin. An HTTP proxy integration closes that gap and mirrors API Gateway’s HTTP integration concept (emulator fidelity for local inner-loop).

Proposed Solution

• Extend emulator route config with IntegrationType (Lambda | Http) and reuse Endpoint for the HTTP target when Http.
• Validate that Http routes have an absolute http(s) endpoint.
• Keep Lambda translation as the default; no change for existing configs.
• Tests covering HTTP proxy passthrough (status, headers, body) vs Lambda path.
  Out of scope for this request:
• gRPC / HTTP/2 trailers (same “share emulator origin” idea later; needs HTTP/2 passthrough, not JSON marshalling).
• AspNetCoreServer null-safety / incomplete REST RequestContext — tracked separately in API Gateway emulator builds incomplete APIGatewayProxyRequest; AspNetCoreServer NREs on null Path / RequestContext / headers #2566 / fix: complete emulator REST APIGatewayProxyRequest and null-safe AspNetCoreServer marshalling #2567.

Other Information

Happy to contribute a PR once this feature request is triaged. We already prototyped IntegrationType=Http on a fork for local use.

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

AWS .NET SDK and/or Package version used

Amazon.Lambda.TestTool 0.15.1

Targeted .NET Platform

.NET 10

Operating System and version

Windows 11