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 — 219 test files and 2,668 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.

[Pybsama]

This is now ready for review.

The branch has been rebased onto the current main (0f17c393) as a single GitHub-verified commit. The three affected files had no upstream changes, and the published tree exactly matches the locally verified tree.

Fresh verification with Node 24.18.1 and pnpm 11.9.0:

• pnpm test useRestoreGraphSession — 1/1 passed
• pnpm checks — lint, formatting, and all workspace type checks passed
• pnpm test — 219/219 test files and 2,668/2,668 tests passed
• git diff --check passed

The new Unit Tests and Docker workflow runs are both action_required with zero jobs, so they are waiting for maintainer approval before execution; this is not a test failure.