Testing: investigate removing PUT /lambda-endpoint from the web runner #736

The web runner (dex-local-runner start-server) exposes PUT /lambda-endpoint. It repoints the Lambda client the runner uses to invoke handlers.

No caller is known. sam passes --lambda-endpoint on the runner's command line at startup and never calls the route. The JS SDK, the JS testing library and the conformance tests do not call it. The route is not documented. It arrived in the testing package on 2025-10-07 with no consumer named.

After #731 the route moves the endpoint only. The runner emulates one region, fixed at startup, and a RegionName other than that region is rejected with InvalidParameterValueException.

Proposal: remove the route, or document who needs it and why. If you call this route, please say so here.