Clean up bot review findings from #216: test helper panic, rollback test gap, NAV underlying-denom guard, migration iteration

[nullpointer0x00]

Follow-ups from bot review feedback on #216 (the epic-branch → main merge PR). These were verified as valid against the current code; consolidating them here rather than patching the merge PR.

1. types/suite_test.go — nil panic in shared assertion helper

RunValidateBasicTable calls assert.Error(t, err, ...) and then unconditionally dereferences err.Error(). Unlike require, assert does not halt the test, so a nil error panics and masks the real assertion failure.

Fix: guard the Contains behind the assert.Error result:

if tc.expectedErr != "" {
    if assert.Error(t, err, "expected error for case %q", tc.name) {
        assert.Contains(t, err.Error(), tc.expectedErr, "error should contain expected substring for case %q", tc.name)
    }
}

2. keeper/vault_test.go — rollback test never reaches NAV seeding

TestCreateVault_InitialPaymentNAVRolledBackOnFailure pre-creates the share marker, so CreateVault fails in createVaultMarker before seedInitialPaymentNAV runs. The NAV is never written, so asserting its absence proves nothing about rollback.

Note: seedInitialPaymentNAV is currently the last step before write(), so no downstream failure can occur after seeding without a test hook. The pragmatic fix is to force a failure inside the seed step (e.g. price denom == payment denom) and assert that the previously created vault account/marker are rolled back — and rename/redocument the test to describe what it actually covers.

3. keeper/nav.go — validateVaultNAVFields permits NAV entries for the underlying asset denom

The guard rejects nav.Denom == vault.TotalShares.Denom but not nav.Denom == vault.UnderlyingAsset. UnitPriceFraction short-circuits to (1,1) for the underlying asset before any NAV lookup, so such an entry can be stored but is never read — dead state that could confuse operators inspecting the NAV table.

Fix: add an explicit rejection for nav.Denom == vault.UnderlyingAsset (plus a test case), and update the SetVaultNAV godoc.

4. keeper/migrations.go — migrations iterate all auth accounts instead of the Vaults collection

Both MigrateVaultAccountPaymentDenomDefaults and migrateInternalNAVSeedFromMarker use k.AuthKeeper.GetAllAccounts(ctx), which is O(total_accounts) in time and memory. The keeper already has a Vaults collection with an O(vault_count) iteration path (GetVaults / Vaults.Walk in keeper/state.go).

Fix: iterate k.GetVaults(ctx) and load each vault via k.GetVault(ctx, addr). One-time migrations, so low urgency — but cheap to fix and more consistent with the rest of the keeper.

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Origin: CodeRabbit and Greptile review threads on #216. Items 5–7 from that review were declined (table-driven refactor preferences; one finding factually incorrect about cache-context event propagation).