fix: add Flickr date string fallback to TimestampToDateTimeConverter

- Try parsing "yyyy-MM-dd HH:mm:ss" format when Unix timestamp fails
- Re-enable DateTaken assertion in PhotoDescriptionIsDeserialized test

Author: toralux

src/Flickr.Net.Test/Entities/PhotoTests.cs

@@ -36,9 +36,7 @@ public void PhotoDescriptionIsDeserialized()
         Assert.False(result.HasError);
         var photo = result.Content.Values[0];
         Assert.Equal("A test description", photo.Description.Content);
-        // Note: DateTaken currently returns default due to TimestampToDateTimeConverter
-        // not supporting Flickr's "YYYY-MM-DD HH:mm:ss" date format.
-        // Assert.Equal(new DateTime(2024, 3, 15, 14, 30, 0), photo.DateTaken);
+        Assert.Equal(new DateTime(2024, 3, 15, 14, 30, 0), photo.DateTaken);
     }
 
     [Fact]

src/Flickr.Net/Internals/JsonConverters/TimestampToDateTimeConverter.cs

@@ -32,6 +32,11 @@ public override DateTime Read(ref Utf8JsonReader reader, Type typeToConvert, Jso
         }
         catch (FormatException)
         {
+            if (DateTime.TryParseExact(value, "yyyy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture, DateTimeStyles.None, out var parsed))
+            {
+                return parsed;
+            }
+
             return DateTime.MinValue;
         }
     }