test: validate default wallet holdings ordering by total value

[techAlhaji]

Closes #516

Summary

Adds an integration test to verify that the wallet holdings endpoint returns holdings sorted by total holding value (balance × price) in descending order when no explicit sort parameter is provided.

Changes

• Added an integration test for GET /wallets/:address/holdings
• Seeded a wallet with three creator holdings having different total values
• Called the endpoint without a sort parameter
• Verified the response is ordered by total holding value in descending order
• Confirmed the highest-value holding appears first and the lowest-value holding appears last

Acceptance Criteria Covered

• ✅ Default sort is total holding value descending
• ✅ Highest-value entry appears first
• ✅ Lowest-value entry appears last

Testing

Added coverage for:

• default holdings ordering
• descending total value sorting
• deterministic ordering using fixed seed data
• regression protection for future sorting changes

Verification

Executed:

• npm test
• npm run build (if applicable)

Notes

• Test-first implementation.
• If the existing implementation does not satisfy the expected behavior, only the minimal production change required to make the endpoint sort by total holding value descending by default should be introduced.
• No unrelated refactoring.

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