MediaFileUnreadable is documented more narrowly than it behaves

The 0.7.0 changelog describes `MediaFileUnreadable` as a media file that "exists but fails *mid-read* while its multipart body is streamed". The Kotlin executor raises it in a wider set of cases.

The actual split is by phase rather than by open-vs-read:

- `MediaFileNotFound` is the pre-serialization check, at multipart-build time.
- `MediaFileUnreadable` covers everything once OkHttp starts writing the body — including a failure to *open* the file at that point (`WpRequestExecutor.kt:437-442`, where `file.source()` throwing `IOException` becomes `MediaFileUnreadableException`), not only a read failure partway through (`:450-458`).

So a file deleted between the multipart build and the body write is `MediaFileUnreadable` despite never having been opened.

Worth aligning the doc comment and changelog with the phase-based split.