test: Add edge case test for addQueryParam URL parse error

Add test case for URL containing control character (null byte)
which triggers url.Parse error and returns path unchanged.

Coverage improved from 88.9% to 100%.

internal/api/oidc_handlers_test.go

@@ -187,6 +187,15 @@ func TestAddQueryParam(t *testing.T) {
 			value: "bar",
 			want:  "/page?foo=bar#section",
 		},
+
+		// Invalid URL (control character causes parse error)
+		{
+			name:  "path with control character returns unchanged",
+			path:  "/page\x00invalid",
+			key:   "foo",
+			value: "bar",
+			want:  "/page\x00invalid",
+		},
 	}
 
 	for _, tc := range tests {