Serializing custom `TableSource` implementations fails

### Describe the bug

`LogicalExtensionCodec` allows providing a custom serialization strategy for a `TableProvider`, but the calling code always expects a `DefaultTableSource` to unwrap:

https://github.com/apache/datafusion/blob/8d7b11b1cdf04aa9e7c402b5c97e45240e6e71a0/datafusion/proto/src/logical_plan/mod.rs#L1004

Which calls `source_as_provider`:

https://github.com/apache/datafusion/blob/8d7b11b1cdf04aa9e7c402b5c97e45240e6e71a0/datafusion/catalog/src/default_table_source.rs#L99-L110

It would be better if `LogicalExtensionCodec` had `encode_table_source` instead of `encode_table_provider`.

### To Reproduce

_No response_

### Expected behavior

_No response_

### Additional context

_No response_

Start in datafusion/proto/src/logical_plan/mod.rs around line 1004 and trace the LogicalExtensionCodec call into datafusion/catalog/src/default_table_source.rs lines 99-110. Confirm how custom TableSource implementations reach source_as_provider, then verify that serialization no longer requires a DefaultTableSource unwrap and that the relevant proto or catalog tests cover the custom case.

Written by the indexing model from the issue text.