fix: remove critical cache failure in oras GetBlobContent

[binbin-li]

Description

What this PR does / why we need it:

In a previous PR, we have removed critical cache failure in oras getReferenceManifest method.
This PR will address the same issue in the GetBlobContent method.
And added more unit tests to fully cover both methods.

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):

Fixes #1741

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Helm Chart Change (any edit/addition/update that is necessary for changes merged to the main branch)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also list any relevant details for your test configuration

• Test A
• Test B

Checklist:

• Does the affected code have corresponding tests?
• Are the changes documented, not just with inline documentation, but also with conceptual documentation such as an overview of a new feature, or task-based documentation like a tutorial? Consider if this change should be announced on your project blog.
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have appropriate license header?

Post Merge Requirements

• MAINTAINERS: manually trigger the "Publish Package" workflow after merging any PR that indicates Helm Chart Change

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files	Coverage Δ
pkg/referrerstore/oras/mocks/oras_storage.go	94.11% <100.00%> (+0.78%)	⬆️
pkg/referrerstore/oras/oras.go	80.97% <100.00%> (+9.30%)	⬆️

... and 1 file with indirect coverage changes

[susanshi]

overall looks good, had some minor suggestions

[susanshi]

LGTM. thanks Binbin