Add store-agnostic projection dead-letter count read to IEventDatabase (#356)

[jeremydmiller]

Closes #356.

What changed

Adds a store-agnostic way to read projection/subscription dead-letter counts to JasperFx.Events.IEventDatabase:

Task<long> CountDeadLetterEventsAsync(ShardName shard, CancellationToken token = default);
Task<IReadOnlyList<DeadLetterShardCount>> FetchDeadLetterCountsAsync(CancellationToken token = default);

public record DeadLetterShardCount(string ProjectionName, string ShardKey, long Count);

DeadLetterShardCount is keyed to align with ShardName.Name (ProjectionName) and ShardName.ShardKey (ShardKey), matching how DeadLetterEvent records them. FetchDeadLetterCountsAsync mirrors the "give me every row" shape of the existing AllProjectionProgress.

Default stand-in implementations

Both members ship with default interface implementations that return 0 / an empty list, so:

• existing IEventDatabase implementers keep compiling with no changes, and
• consumers (e.g. CritterWatch via GetServices<IEventDatabase>()) get a graceful no-op until a store implements the real read.

Real implementations are tracked in JasperFx/marten#4546 (Marten — DeadLetterEvent is already a stored document) and JasperFx/polecat#146 (Polecat — note its dead-letter storage is currently a no-op, so it needs storage first).

Why

JasperFx.Events 2.0 made SkipApplyErrors = true the default: a failed Apply() is recorded as a DeadLetterEvent and the shard keeps advancing, so accumulating dead-letter rows is the primary "this projection is unhealthy" signal. Until now the abstraction only let you write them.

Tests

EventTests/Daemon/DeadLetterCountDefaultsTests — a bare IEventDatabase that overrides neither new member; the calls exercise the default stand-ins and assert 0 / empty (green on net9.0 + net10.0).