#1220 Flake batch 29: rebind diagnostics test retries the non-blocking job snapshot

Main's run 41117835406 (commit d3b75e87) failed subc::tests::rebind_wait_diagnostics_name_active_job_without_waiting_on_database_slot. The diagnostic printed root jobs (None means scheduler locked): None: the executor's root-job snapshot is a non-blocking read and reports None while the scheduler holds its lock, by design, so a single read can miss the held writer.

The test now retries the diagnostic for up to 10 s while it still holds the database slot, until the snapshot gets through. Every assertion is unchanged. It's the same class as the startup-deadline test fixed in #1212.

Test-only. Checked on the build server: the test passed 3 times, formatting and the Windows check passed.

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Summary by cubic

Fixes the flaky rebind_wait_diagnostics_name_active_job_without_waiting_on_database_slot test.

The diagnostic read the executor's root-job snapshot once, and that snapshot reports None while the scheduler holds its lock. The test now retries the diagnostic for up to 10 s while the database slot is held, so the assertions check snapshot contents rather than lock timing. All assertions are unchanged; the change is test-only, with a slice descriptor for the flake batch.