Fix: Encapsulate v2/objectstore package to prevent external usage (#12439)

[suhas-developer07]

Summary

This PR addresses issue #12439 by moving the v2/objectstore package into an internal directory.
This ensures the package cannot be imported or used outside the v2 execution engine, aligning with the intended design boundaries.

What This PR Changes

• Moved backend/src/v2/objectstore → backend/src/v2/internal/objectstore
• Updated all internal imports accordingly
• Ensured tests were moved and still function under the new path
• Removed unused references to the old public package path

Why This Fix Is Necessary

Encapsulating the package ensures:

• A cleaner internal API boundary
• Prevention of accidental external dependencies
• Better long-term maintainability of the v2 execution engine

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[google-oss-prow]

Hi @suhas-developer07. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[hbelmiro]

@suhas-developer07 thank you for working on this.
Can you please sign off your commits and rebase?

[hbelmiro]

/ok-to-test