Guard discovery skip table against shutdown race

[ream88]

The discovery skip table is a named ETS table created in LifecycleManager.init/1, so it is owned by the manager process and is deleted when that process stops. Discovery itself runs in a Task.Supervisor.async_nolink/2 task that is not linked to the manager, and the Task.Supervisor is a sibling started before the manager in a :one_for_all tree, so it is still alive while the manager terminates. terminate/2 does not cancel the in-flight task either.

When the manager stops mid-round the orphaned discovery workers keep running and raise on the next skip-table write:

** (ArgumentError) errors were found at the given arguments:
  * 1st argument: the table identifier does not refer to an existing ETS table (stdlib 8.0) :ets.insert(:"durable_server_discovery_skip_...", {...}) (durable_server 0.1.4) lib/durable_server/lifecycle_manager.ex:2079: anonymous fn/3 in DurableServer.LifecycleManager.discover_and_restart_servers/1 (elixir 1.20.0) lib/task/supervised.ex:105: Task.Supervised.invoke_mfa/2 (elixir 1.20.0) lib/task/supervised.ex:40: Task.Supervised.reply/4

Guard the two writes the same way discovery_diag_snapshot/1 already guards the diagnostics table, and add a regression test that blocks a discovery worker inside get_object/3, kills the manager, then releases the worker.

[Copilot]

Pull request overview

This PR fixes a shutdown race where an orphaned discovery task can outlive DurableServer.LifecycleManager and attempt to write to the manager-owned, named ETS “discovery skip” table after it has been deleted, causing :ets.insert/2 to raise.

Changes:

• Wrap discovery skip-table writes behind a guard (skip_cache_put/2) that checks table existence (and safely ignores ArgumentError) before inserting.
• Add a regression test that blocks a discovery worker inside get_object/3, kills the manager mid-round, then unblocks the worker to ensure no crash occurs on skip-table writes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/durable_server/lifecycle_manager.ex	Replaces direct :ets.insert/2 calls with skip_cache_put/2 to avoid crashes when the skip table is deleted during shutdown.
test/durable_server/lifecycle_test.exs	Adds a PauseOnFetchBackend and a regression test reproducing the shutdown race to validate the guard.

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