test(client): cover protocol_version_override's unknown-version branch

cubic flagged the hint expression's else branch (neither handshake-era
nor modern) as uncovered, which this repo's CI enforces at 100% branch
coverage. Existing tests only exercised the modern-version-override and
mode-pin-conflict branches.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: STiFLeR7

tests/client/test_client.py

@@ -157,6 +157,13 @@ def test_client_rejects_modern_protocol_version_override(app: MCPServer):
         Client(app, mode="auto", protocol_version_override=LATEST_MODERN_VERSION)
 
 
+def test_client_rejects_unknown_protocol_version_override(app: MCPServer):
+    """A `protocol_version_override` that is neither a handshake-era nor a modern version
+    is rejected with no extra hint, unlike the modern-version case above."""
+    with pytest.raises(ValueError, match=r"protocol_version_override must be one of .*got '1999-01-01'$"):
+        Client(app, mode="auto", protocol_version_override="1999-01-01")
+
+
 def test_client_rejects_protocol_version_override_with_a_version_pin_mode(app: MCPServer):
     """`protocol_version_override` has no effect once `mode` already pins a version, so it's
     rejected at construction instead of being silently ignored."""