`AbstractIoUringStreamChannel` only handles `DefaultFileRegion` (via
splice). Any other `FileRegion` implementation hits
`UnsupportedOperationException` from the parent's
`filterOutboundMessage`.

This is inconsistent with EPOLL and NIO, which accept any `FileRegion`
via a `transferTo` fallback. Frameworks like Apache Spark wrap
`DefaultFileRegion` inside composite `FileRegion` implementations (e.g.,
`MessageWithHeader`) that work on NIO and EPOLL but crash on io_uring.

See #16570

For generic `FileRegion` (non-`DefaultFileRegion` or when splice is
unsupported), let it pass through `filterOutboundMessage` into the
outbound buffer, then convert it to a direct `ByteBuf` in **64 KB
chunks** via `FileRegion.transferTo` in the write path
(`scheduleWriteSingle` / `writeComplete0`).

- `DefaultFileRegion` + splice path is unchanged.
- Data is chunked to limit memory usage — each chunk is read via
`transferTo` into a capped `ByteBufWritableByteChannel`, then submitted
as an io_uring async send.
- Partial sends and EAGAIN preserve the chunk buffer for retry;
unrecoverable errors release it and fail the write.
- Empty FileRegions (count = 0) submit a 0-byte send so the completion
path can remove them.

Also enabled `testCustomFileRegion` in `IoUringSocketFileRegionTest` and
`IoUringBufferRingSocketFileRegionTest`.

Any `FileRegion` implementation is now writable on io_uring, consistent
with EPOLL and NIO.

| Transport | `DefaultFileRegion` | Generic `FileRegion` |
|---|---|---|
| NIO | `FileChannel.transferTo` | `region.transferTo` |
| EPOLL | native sendfile | `region.transferTo` fallback |
| io_uring (before) | splice | **UnsupportedOperationException** |
| io_uring (after) | splice (unchanged) | chunked `transferTo` → ByteBuf
→ async send |

---------

Co-authored-by: Norman Maurer <norman_maurer@apple.com>