Public API to enumerate node-owned databases under managed event subscription distribution (#3340)

[jeremydmiller]

Closes #3340.

Problem

Under a MultiTenantedWithShardedDatabases Marten store with
UseWolverineManagedEventSubscriptionDistribution = true, agent assignment is
database-affine (#486/#487): each node only runs the projection/subscription
daemon for the shard databases it owns. But there was no public API for
application code to ask "which databases does the local node own?"

With one physical Postgres database per shard (the reporter runs ~490), a per-pod
component that needs to inspect only its own shards was forced to iterate all
databases — so every pod opened a connection pool to every shard and exhausted a
shared server (53300: remaining connection slots are reserved...). The only
workaround re-implemented the internal EventSubscriptionAgentFamily.DatabaseKeyOf
agent-URI grammar by hand.

Change

• EventSubscriptionAgentFamily.DatabaseIdOf(Uri) — public, robust extraction
  of the shard DatabaseId from an event-subscriptions:// agent URI. The URI
  grammar stays in the family (mirrors the existing UriFor/BuildAgentAsync
  pair) instead of leaking to callers. Returns null for non-scheme or malformed
  URIs.
• IAgentRuntime.AllLocallyOwnedDatabaseIds() — the distinct DatabaseId set
  whose event-subscription agents are currently running on this node. Replaces
  the entire fragile workaround with one call.

The reporter's readiness gate becomes:

var owned = runtime.Agents.AllLocallyOwnedDatabaseIds();
foreach (var db in databases.OfType<IMartenDatabase>()
    .Where(d => owned.Contains(d.Identity)))
{
    var progress = await db.AllProjectionProgress(ct); // only this node's shards
}

Tests

• Fast unit tests for DatabaseIdOf: per-tenant URI, realistic dotted-host sharded
  id round-trip, non-scheme URI, and short-URI null cases.
• End-to-end assertion added to the existing two-database affine-colocation
  multi-node integration test: each node reports exactly its one owned database,
  matching its running agents, and the two nodes partition the full set.

Both green locally (CoreTests + MartenTests against docker Postgres).

Notes / out of scope

The issue's "bonus" ask — a node-scoped projection-progress query — is store-specific
(Marten) and larger; this PR delivers the small, additive, transport-agnostic core
API that unblocks it. A caller can already build that query by intersecting
AllLocallyOwnedDatabaseIds() with the store's databases as shown above.

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