## What changed
Add `write_file_stream` to `ExecutorFileSystem` and `EnvironmentAccess`, preserving the accessor's captured sandbox permissions. Local and streaming-capable remote filesystems replace files one bounded chunk at a time and return the byte count. Older executors buffer the input before sending a single `writeFile` request.
Close remote handles on completion or failure, perform best-effort cleanup after cancellation, and invalidate pending metadata lookups. Document that failure or cancellation can leave a truncated or partial file and that in-flight I/O may outlive cancellation.
## Testing
Add coverage for sequential writes, binary and empty files across local and remote filesystems, source failures, handle cleanup, metadata invalidation after cancellation, and compatibility with older executors.
GitOrigin-RevId: d3eb44e0ac2f0f336e9e157d686464f4d45a3f26
Adam Perry @ OpenAI
a45b8ccb01