There is a mismatch between the documentation for config-settings and the JSON schema.
The documentation has this example:

[tool.uv]
config-settings = { editable_mode = "compat" }

But it fails to validate with uv.schema.json.
On the other hand, the JSON schema does validate this file:

[tool.uv]
config-settings = { single_mode = { String = "compat" }, multi_modes = { List = [ "one", "two"] } }