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 🛠️

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[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.

[diegomarquezp]

@Sboti03 Thank you for your contribution.

The lint checks are failing, would you mind reformatting the affected files?

org.gradle.api.tasks.TaskExecutionException: Execution failed for task ':jib-core:verifyGoogleJavaFormat'.