default FileSource::supports_repartitioning to false

### Describe the bug

This seems like an odd thing to default to `true`. I don't know any formats other than csv which would benefit.

I had a regression when upgrading to df52 where my files (which are a custom format) were getting split up randomly and it took a while to realize this new setting was causing it.

### To Reproduce

_No response_

### Expected behavior

_No response_

### Additional context

_No response_

Start by locating FileSource::supports_repartitioning and inspect how its default is defined and consumed. Confirm the behavior for custom file formats and identify the existing tests covering repartitioning. Done means the default is false while formats that need repartitioning can still opt in, with the relevant tests passing.

Written by the indexing model from the issue text.