feat: support Lambda response streaming end-to-end

[alexis779]

Summary

This PR is the RIE half of the local Lambda response-streaming fix tracked by #175 (and the SAM CLI side aws/aws-sam-cli#6501).

Most of the design rationale, end-to-end test plan, and validation evidence live in the companion SAM CLI PR: aws/aws-sam-cli#9028. Please review the two together.

What this changes

The non-direct invoke path (the one SAM CLI hits over /2015-03-31/functions/function/invocations) was:

1. io.ReadAll-ing the runtime's /response body in rapidcore.Server.sendResponseUnsafe before writing a single slab to the caller, and
2. Storing that slab in a []byte field inside rie.ResponseWriterProxy until rie.InvokeHandler did one final w.Write(invokeResp.Body).

Both layers had to drain before the caller saw any byte, which made Server-Sent Events and any other Lambda response-streaming workload impossible to forward through RIE. This PR wires the real http.ResponseWriter end-to-end:

• internal/lambda/rie/util.go — ResponseWriterProxy now optionally wraps an underlying http.ResponseWriter. On the first Write it copies staged headers / status onto the underlying writer, then forwards every Write straight through and calls Flush() if the underlying supports http.Flusher. The legacy body buffer is kept for pre-stream error paths in InvokeHandler.
• internal/lambda/rapidcore/server.go — sendResponseUnsafe detects streaming via additionalHeaders["Lambda-Runtime-Function-Response-Mode"] (matched case-insensitively, since the runtime sends streaming while the interop constant is Streaming) and uses a new streamingCopy helper. The helper uses a tiny 4 KiB buffer and an explicit Flush after every Write so SSE frames a few dozen bytes long are not coalesced by Go's default 32 KiB io.Copy buffer. The buffered (legacy) branch is preserved and continues to enforce interop.MaxPayloadSize.
• internal/lambda/rie/handlers.go — wires the real w into the proxy and skips the trailing WriteHeader/Write once streaming has begun. The error code paths that re-emit invokeResp.Body (ErrInitDoneFailed, ErrInvokeDoneFailed) are gated on the new Started flag so we never try to overwrite an in-flight stream.

Test plan

go test ./internal/lambda/...

All existing tests pass. End-to-end validation (Node.js handler using awslambda.streamifyResponse, the RIE binary built from this branch, SAM CLI's start-api consuming it, and a browser EventSource) is documented in aws/aws-sam-cli#9028 — the 10 SSE frames arrive at the browser at ~1 second intervals rather than all at once at the end, with the JSON HTTP-integration prelude correctly forwarded to the caller via the Content-Type and Lambda-Runtime-Function-Response-Mode headers.

Notes

• This only touches the non-direct (SAM/RIE) code path. The direct-invoke streaming path in internal/lambda/core/directinvoke is unchanged.
• No new public API; the change is fully transparent for clients that don't use streaming.

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