fix(proxy): start on consoles that cannot encode the startup banner

show_banner is the first thing run_server does, and the banner is box-drawing
characters. On a console whose encoding cannot represent them, which is the default
on Windows, click.echo raises UnicodeEncodeError and the proxy exits with
"Application startup failed" before it binds a port. Setting PYTHONIOENCODING=utf-8
works around it.

The banner is decorative, so fall back to a plain "LiteLLM" when the stream cannot
encode it rather than taking the server down with it.

litellm/proxy/common_utils/banner.py

@@ -15,3 +15,5 @@ def show_banner():
         click.echo(f"\n{LITELLM_BANNER}\n")
     except ImportError:
         print("\n")  # noqa: T201
+    except UnicodeEncodeError:
+        print("\n   LiteLLM\n")  # noqa: T201

tests/test_litellm/proxy/common_utils/test_banner.py

@@ -0,0 +1,36 @@
+import io
+import os
+import sys
+from contextlib import redirect_stdout
+
+sys.path.insert(0, os.path.abspath("../../../.."))
+
+from litellm.proxy.common_utils.banner import show_banner
+
+
+def _cp1252_stdout() -> io.TextIOWrapper:
+    """A stdout that cannot encode the banner, like a Windows console on the default code page."""
+    return io.TextIOWrapper(io.BytesIO(), encoding="cp1252", newline="")
+
+
+class TestShowBanner:
+    def test_show_banner_survives_a_stdout_that_cannot_encode_it(self):
+        """The banner is decorative; an unencodable console must not stop the proxy from starting."""
+        stream = _cp1252_stdout()
+
+        with redirect_stdout(stream):
+            show_banner()
+
+        stream.flush()
+        printed = stream.buffer.getvalue().decode("cp1252")
+        assert "LiteLLM" in printed
+
+    def test_show_banner_prints_the_full_banner_when_stdout_can_encode_it(self):
+        stream = io.TextIOWrapper(io.BytesIO(), encoding="utf-8", newline="")
+
+        with redirect_stdout(stream):
+            show_banner()
+
+        stream.flush()
+        printed = stream.buffer.getvalue().decode("utf-8")
+        assert "██" in printed