Fix oci-spec update outputs pretty manifests

With oci-spec 0.6.6 [#166](youki-dev/oci-spec-rs#166)
the `to_string()` method started to return the
blanket impl of `ToString` due to the `Display` Trait being added.

I'm not entirely sure why I got the display trait instead of the
`to_string()` implemented on `ImageManifest`, in the rust playground I'm
unable to reproduce it. Instead I now call serde_json directly so I
don't have to rely on whatever oci-spec implements as the serialization.

Cargo.toml

@@ -21,7 +21,7 @@ codegen-units = 1
 [dependencies]
 bytes = "1.7.1"
 exitcode = "1.1.2"
-oci-spec = { version = "0.6.4", default-features = false, features = ["image", "distribution"] }
+oci-spec = { version = "0.6.8", default-features = false, features = ["image", "distribution"] }
 url = "2.3.1"
 regex = "1.10.6"
 tracing = "0.1.40"

src/app.rs

@@ -629,13 +629,15 @@ mod tests {
             server
                 .mock("PUT", "/v2/mockserver/foobar/manifests/sha256:7ffd96d9eab411893eeacfa906e30956290a07b0141d7c1dd54c9fd5c7c48cf5")
                 .match_header("Content-Type", "application/vnd.oci.image.manifest.v1+json")
+                .match_body(r#"{"schemaVersion":2,"mediaType":"application/vnd.oci.image.manifest.v1+json","artifactType":"application/pyoci.package.v1","config":{"mediaType":"application/vnd.oci.empty.v1+json","digest":"sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a","size":2},"layers":[{"mediaType":"application/pyoci.package.v1","digest":"sha256:b7513fb69106a855b69153582dec476677b3c79f4a13cfee6fb7a356cfa754c0","size":22}]}"#)
                 .with_status(201) // CREATED
                 .create_async()
                 .await,
             // PUT request to create Index
             server
                 .mock("PUT", "/v2/mockserver/foobar/manifests/1.0.0")
                 .match_header("Content-Type", "application/vnd.oci.image.index.v1+json")
+                .match_body(r#"{"schemaVersion":2,"mediaType":"application/vnd.oci.image.index.v1+json","artifactType":"application/pyoci.package.v1","manifests":[{"mediaType":"application/vnd.oci.image.manifest.v1+json","digest":"sha256:7ffd96d9eab411893eeacfa906e30956290a07b0141d7c1dd54c9fd5c7c48cf5","size":422,"platform":{"architecture":".tar.gz","os":"any"}}]}"#)
                 .with_status(201) // CREATED
                 .create_async()
                 .await,

src/pyoci.rs

@@ -568,11 +568,11 @@ impl PyOci {
             Manifest::Index(index) => {
                 let version = version.context("`version` required for pushing an ImageIndex")?;
                 let url = build_url!(&self, "v2/{}/manifests/{}", name, version);
-                let data = index.to_string();
+                let data = serde_json::to_string(&index)?;
                 (url, data, "application/vnd.oci.image.index.v1+json")
             }
             Manifest::Manifest(manifest) => {
-                let data = manifest.to_string();
+                let data = serde_json::to_string(&manifest)?;
                 let sha = <Sha256 as Digest>::digest(&data);
                 let digest = format!("sha256:{}", hex_encode(&sha));
                 let url = build_url!(&self, "/v2/{}/manifests/{}", name, &digest);