feat(open-sse): early-peek stream error detection + fix UTF-8 loss in CommandCode peek

- new open-sse/utils/streamErrorPeek.js: bounded peek of the first bytes of a
  200 stream; configured pattern match → 502 so account/combo fallback can run
  before any byte reaches the client (streaming included). Re-emits RAW bytes
  so split multi-byte UTF-8 sequences survive the peek (never re-encode
  decoded text — TextDecoder flush corrupts a lone leading byte to U+FFFD).
- chatCore: run the peek after executor.execute when the provider has
  streamErrorPatterns configured; chat.js passes the settings through.
- commandcode executor: same raw-bytes fix in peekForUpstreamError + regression
  test that fails against the old flush-based re-encode.

open-sse/executors/commandcode.js

@@ -27,7 +27,7 @@ export class CommandCodeExecutor extends BaseExecutor {
 		super("commandcode", PROVIDERS.commandcode);
 	}
 
-	transformRequest(model, body, stream, credentials) {
+	transformRequest(_model, body, _stream, _credentials) {
 		body.stream = true;
 		return body;
 	}
@@ -131,13 +131,17 @@ export async function peekForUpstreamError(
 ) {
 	const reader = originalResponse.body.getReader();
 	const decoder = new TextDecoder();
-	const encoder = new TextEncoder();
 	const abortController = new AbortController();
 	const forwardAbort = () => abortController.abort(signal?.reason);
 	if (signal?.aborted) abortController.abort(signal?.reason);
 	else if (signal)
 		signal.addEventListener("abort", forwardAbort, { once: true });
 
+	// Raw bytes for lossless re-emission; decoded text is only used for line
+	// parsing / error detection. Never re-encode decoded text: TextDecoder
+	// holds a split multi-byte char internally and flush() would replace it
+	// with U+FFFD, corrupting the stream.
+	const rawChunks = [];
 	let peeked = "";
 	let errorEvent = null;
 	let committed = false;
@@ -167,6 +171,7 @@ export async function peekForUpstreamError(
 				Math.max(deadline - Date.now(), 1),
 			);
 			if (done) break;
+			rawChunks.push(value);
 			peeked += decoder.decode(value, { stream: true });
 			const lines = peeked.split("\n");
 			// The last segment may be a partial line — only parse complete ones.
@@ -188,10 +193,6 @@ export async function peekForUpstreamError(
 		// the downstream stream pipeline (stall detection, abort handling) takes over.
 	}
 
-	// Flush any partial multi-byte UTF-8 sequence held by the decoder so the
-	// re-encoded peeked bytes round-trip losslessly.
-	peeked += decoder.decode();
-
 	if (signal) signal.removeEventListener("abort", forwardAbort);
 
 	if (errorEvent) {
@@ -210,7 +211,9 @@ export async function peekForUpstreamError(
 		start(controller) {
 			(async () => {
 				try {
-					if (peeked) controller.enqueue(encoder.encode(peeked));
+					// Re-emit RAW bytes (never re-encoded decoded text) so split
+					// multi-byte UTF-8 sequences survive the peek untouched.
+					for (const c of rawChunks) controller.enqueue(c);
 					while (true) {
 						const { done, value } = await reader.read();
 						if (done) break;