"Produce from Document" fails with FileSystemError when source is a Kafka message preview

### Description
When attempting to use the "Produce from Document" command on a message that was opened via the Kafka message preview (using the confluent.topic.message: URI scheme), the operation fails with a FileSystemError.
The extension incorrectly passes the full custom URI (e.g., confluent.topic.message:c:\Users\...\file.json?{...}) to VS Code's filesystem API, which cannot resolve this mixed URI format.

### Sentry links
- [Windows](https://confluent.sentry.io/issues/7170696986/?alert_rule_id=15357085&alert_type=issue¬ification_uuid=ec5ea287-ef91-4a53-a6b4-6e659222d5e4&project=4507583218253824&referrer=slack)
- [macOS](https://confluent.sentry.io/issues/6980806059/?project=4507583218253824&query=Failed%20to%20read%20file%20confluent.topic.message&referrer=issue-stream)