Handle short writes when creating memory files

[sylvesterkaczmarek]

Summary

Ensure the local filesystem memory tool writes complete file contents when os.write() performs a legal short write.

Both BetaLocalFilesystemMemoryTool.create() and BetaAsyncLocalFilesystemMemoryTool.create() currently call os.write() once and ignore its return value before reporting success. POSIX writes are allowed to consume fewer bytes than requested, so a short write can leave a newly created memory file truncated while the tool still returns File created successfully.

The existing atomic edit/insert paths already loop until the complete buffer has been written, so create currently has weaker data-integrity semantics than later updates to the same memory file.

Fix

Introduce a small shared _write_all() helper that retries writes until the complete byte buffer has been persisted and treats a zero-byte write as an error.

Use the helper consistently in synchronous and asynchronous memory creation and in the existing sync/async atomic replace paths.

Regression coverage

Adds deterministic sync and async tests that force os.write() to accept only three bytes per call. The tests use UTF-8 content containing a multibyte character and verify that multiple writes occur and the final file exactly matches the requested content.

The production change is confined to Anthropic's hand-maintained local filesystem memory tool.