Add thorough migration tests for idxdb-store and SqliteStore

[JereSalo]

As noted in #1767 (review), the current migration test scaffolding for idxdb-store mostly verifies that Dexie's version(N).stores().upgrade() mechanism works as promised (schema upgrades, data preservation on reopen). We should add tests that cover more realistic migration scenarios.

• idxdb-store: Extend the vitest + fake-indexeddb suite from idxdb-store: scaffolding for IndexedDB schema migrations #1767 with real data transformations — migrations that reshape existing rows, not just add/remove stores or indexes.
• SqliteStore: Add equivalent migration tests covering data transformations, not just schema DDL.

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.

[WiktorStarczewski]

After the web/WASM split (#1992), the idxdb-store half of this issue is tracked separately at 0xMiden/web-sdk#125. This issue now scopes effectively to the SqliteStore half: real data-transformation migration tests, not just schema DDL.