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.

tests/unit/commandcode-executor.test.js

@@ -98,4 +98,26 @@ describe("commandcode executor — early-error peek", () => {
 		expect(res.status).toBe(200);
 		await res.body.cancel();
 	});
+
+	it("re-emits raw bytes so a multi-byte char split across the peek boundary survives", async () => {
+		// chunk1 ends mid-é (0xC3); the peek commits on the first complete line
+		// (text-delta "hi") while the decoder still holds 0xC3. Re-emission must
+		// use RAW bytes — re-encoding decoded text would replace 0xC3 with U+FFFD.
+		const first = Buffer.from(
+			'{"type":"text-delta","text":"hi"}\n{"type":"text-delta","text":"caf',
+		);
+		const chunk1 = new Uint8Array([...first, 0xc3]);
+		const rest = new Uint8Array([0xa9, 0x22, 0x7d, 0x0a]); // é"}\n
+		const body = new ReadableStream({
+			start(c) {
+				c.enqueue(chunk1);
+				c.enqueue(rest);
+				c.close();
+			},
+		});
+		const res = await peekForUpstreamError(new Response(body, { status: 200 }), "m");
+		const text = await res.text();
+		expect(text).toContain("café");
+		expect(text).not.toContain("\uFFFD");
+	});
 });