fix: Add graceful error handling for ChunkedEncodingError during streaming (#57)

Fixes connection interruptions from ChatGPT API by catching ChunkedEncodingError
and gracefully ending the stream with [DONE] instead of returning 500 errors.

Two-level error handling:
1. Catches errors during stream initialization
2. Catches errors during stream iteration

This prevents abrupt failures when the upstream connection is interrupted mid-response.

chatmock/utils.py

@@ -432,10 +432,23 @@ def sse_translate_chat(
         except Exception:
             return None
     try:
-        for raw in upstream.iter_lines(decode_unicode=False):
+        try:
+            line_iterator = upstream.iter_lines(decode_unicode=False)
+        except requests.exceptions.ChunkedEncodingError as e:
+            if verbose and vlog:
+                vlog(f"Failed to start stream: {e}")
+            yield b"data: [DONE]\n\n"
+            return
+
+        for raw in line_iterator:
+            try:
                 if not raw:
                     continue
-            line = raw.decode("utf-8", errors="ignore") if isinstance(raw, (bytes, bytearray)) else raw
+                line = (
+                    raw.decode("utf-8", errors="ignore")
+                    if isinstance(raw, (bytes, bytearray))
+                    else raw
+                )
                 if verbose and vlog:
                     vlog(line)
                 if not line.startswith("data: "):
@@ -447,8 +460,18 @@ def sse_translate_chat(
                     break
                 try:
                     evt = json.loads(data)
-            except Exception:
+                except (json.JSONDecodeError, UnicodeDecodeError):
                     continue
+            except (
+                requests.exceptions.ChunkedEncodingError,
+                ConnectionError,
+                BrokenPipeError,
+            ) as e:
+                # Connection interrupted mid-stream - end gracefully
+                if verbose and vlog:
+                    vlog(f"Stream interrupted: {e}")
+                yield b"data: [DONE]\n\n"
+                return
             kind = evt.get("type")
             if isinstance(evt.get("response"), dict) and isinstance(evt["response"].get("id"), str):
                 response_id = evt["response"].get("id") or response_id