Expose mediaType on ContentDescriptorTemplate

[Sboti03]

Motivation

ContentDescriptorTemplate stores mediaType but does not expose it,
while other descriptor fields (digest, size, urls, annotations) are accessible
via public getters.

This makes it difficult to work with OCI artifacts without using reflection.

Change

Adds a public getter:

@Nullable
public String getMediaType()

Use case

Needed to inspect layer descriptors and select artifacts based on media type,
for example Helm chart layers: application/vnd.cncf.helm.chart.content.v1.tar+gzip

Notes

• No behavioral change
• Only exposes already parsed data
• Consistent with existing descriptor getters

Fixes #4498 🛠️

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gemini-code-assist]

Code Review

This pull request removes an unused suppression annotation and adds a public getter for the mediaType field within the ContentDescriptorTemplate class. The reviewer recommends adding unit test assertions in V22ManifestTemplateTest and OciManifestTemplateTest to ensure the new getter correctly returns the expected data.