Align supported Config-settings with example in docs (#12947)

Closes #12929
## Summary
Untag the `config-settings` value to support JSON schema according to
the
[docs](https://docs.astral.sh/uv/reference/settings/#config-settings).
```toml
[tool.uv]
config-settings = { editable_mode = "compat" }
```
<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan
Verified using the "Even Better TOML" extension with paths to old and
new `uv.schema.json`.


## Notes
I could not reproduce the issue with either the `taplo` (on which Even
Better Toml is built, afaik) and `check-jsonschema` CLI tools; with both
old and new versions of the `uv.schema.json` validated the
`pyproject.toml`.

Maybe for these there is some additional regularization going on and
that's also how a breaking case ended up in the docs?
I'm unsure on how to test for this.

After about an hour, the Even better TOML VSCode extension was the only
way to reproduce failing validation.

Let me know if I can do something else.

<!-- How was it tested? -->

crates/uv-configuration/src/config_settings.rs

@@ -29,7 +29,7 @@ impl FromStr for ConfigSettingEntry {
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
-#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
+#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema), schemars(untagged))]
 enum ConfigSettingValue {
     /// The value consists of a single string.
     String(String),

uv.schema.json

@@ -645,35 +645,17 @@
       ]
     },
     "ConfigSettingValue": {
-      "oneOf": [
+      "anyOf": [
         {
           "description": "The value consists of a single string.",
-          "type": "object",
-          "required": [
-            "String"
-          ],
-          "properties": {
-            "String": {
-              "type": "string"
-            }
-          },
-          "additionalProperties": false
+          "type": "string"
         },
         {
           "description": "The value consists of a list of strings.",
-          "type": "object",
-          "required": [
-            "List"
-          ],
-          "properties": {
-            "List": {
-              "type": "array",
-              "items": {
-                "type": "string"
-              }
-            }
-          },
-          "additionalProperties": false
+          "type": "array",
+          "items": {
+            "type": "string"
+          }
         }
       ]
     },