Substrait physical plan producer emits invalid URIs for LocalFiles

The Substrait physical plan producer emits bare paths instead of valid URIs for `LocalFiles` items.

https://github.com/apache/datafusion/blob/1f26716280f330554b37791a057aaff44068698e/datafusion/substrait/src/physical_plan/producer.rs#L65-L67

This produces paths like `path/to/file.parquet` instead of proper URIs like `file:///path/to/file.parquet`.

These bare paths fail `Url::parse()` with "relative URL without a base" since they're not valid URIs per RFC 3986.

Start in datafusion/substrait/src/physical_plan/producer.rs at lines 65-67 and inspect how LocalFiles paths are emitted. Verify the resulting values are valid file URIs that Url::parse() accepts, including the file:///path/to/file.parquet form.

Written by the indexing model from the issue text.