fix: Prevent and clean up zero-valued entries in Alpha storage maps during hotkey and coldkey swaps

[bittoby]

Description

Prevent and clean up zero-valued entries in Alpha storage maps during hotkey and coldkey swaps.

Coldkey and hotkey swap logic was inserting zero-valued entries into Alpha, TotalHotkeyShares, TotalHotkeyAlphaLastEpoch, and AlphaDividendsPerSubnet without checking if the value was zero first. These entries persist indefinitely and bloat storage - a single coldkey swap could produce ~18k zero entries, and roughly 52% of Alpha entries on-chain are zero-valued.

This PR adds zero-guards to all insertion points in swap_coldkey.rs and swap_hotkey.rs, and includes a one-time migration to purge existing zero entries from the four affected storage maps.

Related Issue(s)

• Closes Alpha and hotkey storage maps accumulate persistent zero-valued entries #2398

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Breaking Change

No breaking changes.

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have run ./scripts/fix_rust.sh to ensure my code is formatted and linted correctly
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Additional Notes

The migration collects the full Alpha map into memory before removing zero entries. This is intentional - mutating a Substrate storage map while iterating over it is unsafe. The other three maps are smaller and iterated directly. All four maps use HasMigrationRun to ensure idempotency.

[bittoby]

@open-junius Could you please review this PR? Welcome to any feedbacks.
thank you

[open-junius]

change the target branch as devnet-ready, not main.

[bittoby]

I changed target branch. Thanks! Ready to review

[open-junius]

please check https://github.com/opentensor/subtensor/pull/2509/changes, which also prevent some zero value insert. Could you include these storage into the PR.

[bittoby]

@open-junius Thanks for your feedback! I updated all following your feedback. Could you please review again?

[open-junius]

please merge the latest devnet-ready, will fix the e2e test.

[bittoby]

@open-junius I merged the latest devnet-ready. Could you take a look once more?
thank you

[bittoby]

@open-junius All e2e tests passed. thank you. The benchmark testing issue is not relate to my change.