Splits the edit tool into two separate tools `write_file` (previously
`mode = write`), and `edit_file` (previously `mode = edit`).
This makes the JSON schema for the `edit_tool` much simpler. We've seen
models (especially older ones) struggle with providing `mode = edit +
edits` and `mode = write + content` fields. This seems to improve eval
scores for Sonnet 4.6 slightly.

Also added two unit evals to ensure that the model uses the tool to
create new/override existing files

Release Notes:

- N/A

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Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>