Remove redundant DeleteMessages(StoredId, positions) IMessageStore overload

[stidsborg]

Summary

Removes Task DeleteMessages(StoredId storedId, IEnumerable<long> positions) from IMessageStore. Message positions are globally-unique identity values, so the StoredId filter was redundant — the positions-only overload DeleteMessages(IReadOnlyList<long> positions) covers the same need.

• ExistingMessages.Remove (the sole production caller) now routes through the positions-only overload.
• The two-arg overload is removed from the interface, all four store implementations (InMemoryFunctionStore, PostgreSqlMessageStore, MariaDbMessageStore, SqlServerMessageStore), and the now-orphaned SqlGenerator.DeleteMessages(StoredId, …) in the PostgreSQL/MariaDB/SqlServer generators.

Tests

• The shared message-store test templates are migrated to the positions-only overload. Method names are unchanged, so the per-store override files needed no edits. This is a net coverage gain — the positions-only overload previously only had in-memory coverage (via MessageClearer) and is now exercised across all stores.
• DeleteMessagesWithNonExistentPositionsDoesNotThrow was reworked to delete a real, already-removed position rather than a hardcoded 999/1000; without the StoredId filter an arbitrary number could collide with another flow's globally-unique position in a shared database.
• Dropped the throwing stub for the removed method from the ControllableMessageStore test double.

Verification

• ✅ Full solution builds clean (0 warnings, 0 errors)
• ✅ In-memory DeleteMessages* + MessageClearer: 10/10
• ✅ PostgreSQL DeleteMessages*: 5/5
• ✅ MariaDB DeleteMessages*: 5/5
• ✅ SqlServer DeleteMessages*: 5/5

🤖 Generated with Claude Code

Also: flaky test fix (unrelated)

CI surfaced a pre-existing flaky test, PostponingExistingFunctionFromControlPanelSucceeds, while this PR was in review. It postponed to new DateTime(1_000_000) — a year-0001 timestamp that is already expired — so the PostponedWatchdog could resume the function and flip its status Postponed → Executing before the assertions ran. Fixed by postponing to a future timestamp (DateTime.UtcNow.AddDays(1)), matching the sibling PostponingExistingActionFromControlPanelSucceeds test. Not related to the DeleteMessages change, but included here since it was blocking this PR's CI.