Clear unavailable sessions after restore

[Pybsama]

Description

• Persist a successful empty restoration through the existing graph-session update boundary so a stale previous session is replaced by the current empty graph and the restore action disappears.
• Keep failed restorations retryable and leave non-empty restorations on the existing add-and-persist path.
• Add a hook regression test and a DbState session fixture that reproduce a persisted session whose entities no longer exist in the graph database.

Validation

• pnpm test useRestoreGraphSession
• pnpm checks
• pnpm test — 207 test files and 2,328 tests passed

Related Issues

• Fixes Restoring previous graph leaves restore button present #959

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified pnpm checks passes with no errors.
• I have verified pnpm test passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.