Fix migration memory leak: old collection meta doc not deleted (#7791)

[OskarD]

This PR contains:

• A BUGFIX

Describe the problem you have without this PR

Two bugs in the migration-schema plugin combine to create a memory leak (~100 MB/min) that worsens on every app restart (issue #7791).

Bug 1: After startMigration() migrates all documents, it tries to delete the old collection meta doc using a reference cached at the start of the function. If the doc's _rev has changed (due to concurrent writes or storage internals), the writeSingle() fails with a conflict. The conflict handler only ignores already-deleted docs, so the meta doc persists. On every restart, mustMigrate() finds this orphaned doc and returns true.

Bug 2: migratePromise() calls startMigration() without await (fire-and-forget, by design for multi-tab leader election). When combined with Bug 1, a stale DONE status doc from a previous migration run satisfies the Promise.race immediately, while the orphaned startMigration() creates leaked storage instances and subscriptions. The fire-and-forget also means any errors thrown from startMigration() become unhandled promise rejections.

Changes

1. Replace single-attempt meta doc deletion with a conflict-retry loop that re-fetches the doc before each attempt (same pattern used by updateStatus() and addConnectedStorageToCollection()).
2. Add .catch(() => {}) to the fire-and-forget startMigration() call to prevent unhandled promise rejections. Errors still propagate via the status observable.

Todos

• Tests
• Typings

[OskarD]

Note on .catch(() => {}) (line 585)

The original code called this.startMigration(batchSize) as fire-and-forget without await or .catch(). Errors before the inner try/catch (e.g. createStorageInstance(), leader election) were already unhandled promise rejections. The .catch(() => {}) makes this explicit and prevents runtime crashes in strict environments.

A better fix would wrap the entire startMigration() body in the existing try/catch so early failures also set status = ERROR and surface through the status observable. That way migratePromise() would resolve with ERROR instead of hanging. But that is a separate improvement beyond the scope of this PR.