Fix kebab casing of README variants in build backend (#14762)

## Summary

In this context, `rename_all` only applies to the variants, not their
fields.

Closes #14761.

crates/uv-build-backend/src/metadata.rs

@@ -703,7 +703,7 @@ struct Project {
 /// The optional `project.readme` key in a pyproject.toml as specified in
 /// <https://packaging.python.org/en/latest/specifications/pyproject-toml/#readme>.
 #[derive(Deserialize, Debug, Clone)]
-#[serde(untagged, rename_all = "kebab-case")]
+#[serde(untagged, rename_all_fields = "kebab-case")]
 pub(crate) enum Readme {
     /// Relative path to the README.
     String(PathBuf),
@@ -713,7 +713,7 @@ pub(crate) enum Readme {
         content_type: String,
         charset: Option<String>,
     },
-    /// The full description of the project as inline value.
+    /// The full description of the project as an inline value.
     Text {
         text: String,
         content_type: String,
@@ -965,6 +965,65 @@ mod tests {
         "###);
     }
 
+    #[test]
+    fn readme() {
+        let temp_dir = TempDir::new().unwrap();
+
+        fs_err::write(
+            temp_dir.path().join("Readme.md"),
+            indoc! {r"
+            # Foo
+
+            This is the foo library.
+        "},
+        )
+        .unwrap();
+
+        fs_err::write(
+            temp_dir.path().join("License.txt"),
+            indoc! {r#"
+                THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+                INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+                PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+                HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF
+                CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE
+                OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+        "#},
+        )
+        .unwrap();
+
+        let contents = indoc! {r#"
+            # See https://github.com/pypa/sampleproject/blob/main/pyproject.toml for another example
+
+            [project]
+            name = "hello-world"
+            version = "0.1.0"
+            description = "A Python package"
+            readme = { file = "Readme.md", content-type = "text/markdown" }
+            requires_python = ">=3.12"
+
+            [build-system]
+            requires = ["uv_build>=0.4.15,<0.5"]
+            build-backend = "uv_build"
+        "#
+        };
+
+        let pyproject_toml = PyProjectToml::parse(contents).unwrap();
+        let metadata = pyproject_toml.to_metadata(temp_dir.path()).unwrap();
+
+        assert_snapshot!(metadata.core_metadata_format(), @r"
+        Metadata-Version: 2.3
+        Name: hello-world
+        Version: 0.1.0
+        Summary: A Python package
+        Description-Content-Type: text/markdown
+
+        # Foo
+
+        This is the foo library.
+        ");
+    }
+
     #[test]
     fn self_extras() {
         let temp_dir = TempDir::new().unwrap();