Add thorough migration tests for idxdb-store

Split out from 0xMiden/miden-client#1808 as part of the web-sdk repo split.

Extend the vitest + fake-indexeddb suite from 0xMiden/miden-client#1767 with real data transformations — migrations that reshape existing rows, not just add/remove stores or indexes.

Migrations are not actively used yet since the Miden network resets on every upgrade and the client nukes the DB on major/minor version bumps. This becomes important once the network stabilizes and clients need to migrate user data in-place across upgrades.

The SqliteStore counterpart remains tracked at 0xMiden/miden-client#1808.

Look at the existing vitest + fake-indexeddb suite from the linked PR #1767 in the miden-client repo. Understand the migration logic and data structures for idxdb-store. The task is to extend the test suite with real data transformation scenarios, not just schema changes. Start by examining the current migration tests and the store's implementation to see how rows are shaped.

Escrito por el modelo de indexación a partir del texto del issue.