Implement Lambda runtime init error reporting

[joe4dev]

Summary

When a Lambda runtime exits unexpectedly or throws an error during initialization, LocalStack previously received no callback and would wait until the environment timeout. This PR adds two complementary error-reporting paths so LocalStack immediately receives a structured ErrorResponse instead of timing out.

Changes

cmd/localstack/supervisor.go — new

LocalStackSupervisor wraps the sandbox's ProcessSupervisor and intercepts process termination events on a background goroutine. When a runtime-* process exits and the runtime is not already shutting down intentionally, it constructs a FaultData with fatalerror.RuntimeExit and calls eventsAPI.SendFault(). An atomic isShuttingDown flag (set on Terminate/Kill, cleared on Exec) prevents duplicate fault events during graceful restarts.

cmd/localstack/events.go — new

LocalStackEventsAPI wraps telemetry.StandaloneEventsAPI and overrides SendFault to forward runtime faults to LocalStack as error status callbacks. It embeds the requestId in the error message and tracks the current invoke ID (thread-safe via sync.RWMutex) for faults that originate outside of an active invocation.

cmd/localstack/custom_interop.go — modified

• ErrorResponse.RequestId changed from string to *string with omitempty: a pointer-to-empty-string serializes as "" (required by LocalStack's init error contract), while nil (used in fault events) is omitted.
• NewCustomInteropServer now accepts a pre-created *LocalStackAdapter instead of constructing one internally, so the adapter is shared with the events API.
• SendInitErrorResponse now parses the raw error payload from the runtime, enriches it with the current invoke ID, and asynchronously POSTs the structured response to LocalStack /status/{runtime_id}/error before propagating to the delegate. Falls back to the raw payload if parsing fails.

cmd/localstack/main.go — modified

LocalStackAdapter is created once upfront and passed to both NewLocalStackEventsAPI and NewCustomInteropServer. The sandbox is configured with SetEventsAPI(lsEventsAPI) and SetSupervisor(localStackSupv). The supervisor's context is cancelled in the existing shutdown func alongside the file watcher.

Tests

Covered by the integration tests in localstack/localstack-pro#7293:

Scenario	Test
Exception raised during module import	test_lambda_runtime_error
sys.exit() called during init	test_lambda_runtime_exit
Missing AWS_LAMBDA_EXEC_WRAPPER script	test_lambda_runtime_wrapper_not_found

Related

Depends on #101
Closes DEVX-1