Closes #59001

Saving a notebook wrote the file with
`project.fs().atomic_write(abs_path, ...)`. `Project::fs()` is always
the client's own filesystem, so for a remote project the remote path was
resolved against the local machine: on Windows the drive letter got
prepended and the atomic-write temp file used a backslash, producing
`C:/data/project/src_example\.tmpPuzw3d3` and `os error 3`. The same
thing fails on macOS/Linux clients with `os error 2`.

Both `save` and `save_as` now go through the project's buffer machinery
instead: open the buffer for the notebook's `ProjectPath`, set its text
to the serialized notebook, then `save_buffer` / `save_buffer_as`.
Writes are routed over the remote connection for remote projects, and
the open buffer stays in sync locally.

`save_as` also updates the item's project path and entry id, since
`save_buffer_as` moves the buffer to the new path and otherwise the next
save would write back to the old file.

Added a test that edits a cell, saves, and checks that the notebook
buffer held by the project reflects the saved file. It fails against the
old implementation.

Release Notes:

- N/A

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Co-authored-by: Finn Evers <finn.evers@outlook.de>