chore: add stdlib mock for io.Closer and update mockgen install instruction

[aman-coder03]

What

• added mocks/io/mock_closer.go generation to the make mocks target in the Makefile
• updated the mockgen install instruction in the Makefile comment to use the maintained fork (go.uber.org/mock/mockgen@latest) instead of the archived github.com/golang/mock
• promoted github.com/opencontainers/image-spec from indirect to direct dependency in go.mod

Why

mocks/io/mock_closer.go file exists in the repo and is used in tests, but there was no corresponding make mocks entry to regenerate it. This means anyone running make mocks would lose this file with no way to get it back, making the mock effectively unmanageable.
mockgen install instruction pointed to github.com/golang/mock which has been archived since June 2023. The maintained fork is go.uber.org/mock
follows up on the discussion in #564

How to verify

Run make mocks.... mocks/io/mock_closer.go file should now be regenerated correctly alongside all other mocks

[thijmv]

Thank you for the PR, a couple of comments.

[thijmv]

Was this done accidentally?

[thijmv]

Let's revert this change as the project is currently setup to use github.com/golang/mock/mockgen meaning that mocks generated using Uber's fork would not compile without changes to go.mod.

[thijmv]

Please regenerate the mocks and commit the changes.

[aman-coder03]

thanks @thijmv , i will revert the Makefile comment and go.mod changes
for regenerating the mocks....i am on Windows and hitting compatibility issues running make mocks natively (the mockgen reflect mode builds a Linux binary). Could you confirm which version of mockgen and Go you used to generate the current mocks? that will help me match the output exactly

[thijmv]

Go >= 1.24 using github.com/golang/mock/mockgen@v1.6.0.

[thijmv]

Can you update the PR description to accurately reflect the changes you are making? Here are two more possible improvements:

1. Update the install comment to go install github.com/golang/mock/mockgen@v1.6.0.
2. Remove the sed calls from the mocks target, as they are no longer needed.