Fix Polecat projection agents not starting under managed distribution (#3168)

[jeremydmiller]

Closes #3168.

Root cause

Under UseWolverineManagedEventSubscriptionDistribution, a Polecat-backed store's async projection agents surfaced (AllKnownAgentsAsync returned them) but never started — even single-node — with the node reporting no running agents (node N: []). Every start threw:

AgentStartingException: Failed trying to start agent
  event-subscriptions://sqlserver/Polecat/localhost,1434.master/trip/all
  ---> System.ArgumentOutOfRangeException: Unknown event projection or subscription

EventSubscriptionAgentFamily keyed its store map by EventStoreIdentity.ToString() ("{Name}:{Type}", original casing → "Polecat:SqlServer"), but BuildAgentAsync rebuilt the lookup key from the agent URI: $"{uri.Segments[1]}:{uri.Host}". The store Type rides in the URI authority, which System.Uri lowercases → "Polecat:sqlserver" → lookup miss → throw.

Marten was unaffected purely because its Identity.Type is already lowercase ("marten"); it's a latent bug for any provider with an uppercase Type.

Fix

Normalize the store key to one casing (matching System.Uri's authority normalization) at all three sites — the _stores keying, the BuildAgentAsync reverse lookup, and FindStore — via a small documented StoreKey helper in core EventSubscriptionAgentFamily.

Tests

• New Polecat regression test polecat_managed_distribution_starts_agents_3168 asserts the three async projection agents actually run on one node (mirrors Marten's everything_is_started_up_on_one_node). Fails before the fix (30s timeout, node N: []), passes after (~5s).
• Existing Polecat surfacing tests + Marten single/multi-node distribution tests still green — confirms no regression and no double-start (the WolverineProjectionCoordinator path is unchanged and identical to Marten's).
• dotnet build wolverine.slnx -c Release clean.

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