Add V2 streaming Lambda adapter and fix body-less 502

[kieran-sf]

Summary

• Adds createStreamingLambdaAdapterV2, a new streaming Lambda adapter for MRT SSR that uses a socket-intercept design: it drives an unmodified Node http.ServerResponse on a fake net.Socket, parses the raw HTTP wire bytes, and forwards them to the AWS response stream. This avoids monkey-patching ServerResponse (as V1 does), improving compatibility with both Express 4 and 5. Supports br/gzip/deflate response compression with Accept-Encoding negotiation via the new StreamingCompressionConfig type. V1 remains the default.
• Fixes body-less streaming responses returning a 502 at the MRT edge. The AWS runtime (awslambda.HttpResponseStream.from) emits the status/headers metadata prelude lazily — only just before the first write to the stream. A response that reaches end() without ever writing (302 redirects, 204/304, HEAD, bare res.end()) ships with no prelude, so the streaming edge cannot frame it and returns its own 502 InternalServerErrorException regardless of the intended status code. Both adapters now force a zero-length write before ending the stream.

Fix details

• V1 (create-lambda-adapter.ts): adds a body-less branch in res.end() that calls writeChunk(Buffer.alloc(0)), which also flushes any active compressor.
• V2 (create-lambda-adapter-v2.ts): writes an empty string in onReceiveHead, which fires whenever the native ServerResponse flushes its head (always, even for body-less responses).

Testing

• New regression tests in both adapters' test suites use a faithful RIC mock (prelude emitted only on first write) — the default identity from stub cannot catch this bug. Cases: bare res.end(), body-less redirect (asserts 302), 204 No Content, and body-less compressed.
• Verified the tests have teeth: with the fix temporarily disabled, the identity body-less cases fail; restored, the full suite passes (655 passing, 0 failing).
• Typecheck and lint clean for the affected packages; all modified files are prettier-clean.

Changesets

• @salesforce/mrt-utilities minor — V2 adapter.
• @salesforce/mrt-utilities patch — body-less 502 fix.