Simplify GetMessagesForReplica to List<StoredMessages> and add ignorePositions

[stidsborg]

What

Simplifies IMessageStore.GetMessagesForReplica and lays the foundation for de-duplicating watchdog message delivery.

Before

Task<Dictionary<StoredId, List<StoredMessage>>> GetMessagesForReplica(ReplicaId replicaId);

After

Task<List<StoredMessages>> GetMessagesForReplica(ReplicaId replicaId, IReadOnlyList<long> ignorePositions);

• New record StoredMessages(StoredId StoredId, List<StoredMessage> Messages) replaces the dictionary return shape.
• ignorePositions excludes already-handled positions so they aren't re-delivered on subsequent ticks.
• FlowsManager.Push now takes IReadOnlyList<StoredMessages>.

MessageWatchdog (version 1)

The watchdog keeps an ever-growing HashSet<long> of positions it has already pushed and passes it as ignorePositions each tick. This is a deliberate first step — it will be refined once the QueueManager reports the positions it has persisted into its effects, which will bound the set.

Store implementations

Each store filters the ignore-set with a stable, parameterized query so the plan is reused regardless of set size (no plan-cache churn from inlined literals):

Store	Filter
PostgreSQL	position != ALL($2) (bound array param)
SQL Server	Position NOT IN (SELECT CAST(value AS BIGINT) FROM STRING_SPLIT(@IgnorePositions, ','))
MariaDB	FIND_IN_SET(position, ?) = 0
InMemory	HashSet lookup

The empty case is handled per dialect (Postgres empty array; SQL Server -1 sentinel in the parameter to avoid an empty-string CAST; MariaDB FIND_IN_SET(_, '') = 0 is naturally a no-op).

Tests

Adds GetMessagesForReplicaExcludesIgnoredPositions to the shared MessageStoreTests template, run across all four stores (InMemory, PostgreSQL, MariaDB, SQL Server). Covers:

• empty ignore-set returns every message for the replica, grouped by flow and ordered by position;
• messages owned by a different replica are never returned;
• ignoring a position drops just that message;
• ignoring all of a flow's positions drops the whole group.

All four pass against the real databases.