Deduplicate inbox registration and close inbox retention test gaps

[Vulthil]

Summary

• Hoist the duplicated retention/metrics/TimeProvider registration out of AddRelationalInbox
  and AddCosmosInbox into one internal core entry point; the store extensions now register
  only their store type. The configure delegate now runs once eagerly (for the gates) plus
  once deferred (for injected consumers), down from three invocations.
• Remove ValidateDataAnnotations()/[Range] pairs that duplicated adjacent lambda validation
  in both the inbox and outbox option types. On the outbox side this required adding explicit
  lambda checks first for five constraints that were not actually mirrored anywhere else, to
  keep validation behavior unchanged.
• AddInboxRetention gains ArgumentNullException.ThrowIfNull, registers TimeProvider.System, and
  makes its configure parameter optional (PublicAPI.Shipped.txt updated in place; verified via
  build and pack that this is accepted as a compatible change).
• Add the missing inbox retention test coverage: background-service batching/cutoff behavior,
  and DI-registration-level tests (options validation, enabled/disabled hosted-service gating)
  that didn't exist for the inbox family before.
• Hoist a per-delivery typeof(TMessage).FullName computation to a static field in
  IdempotentConsumeFilter, and make the Cosmos retention sweep treat a concurrent sweeper's
  DbUpdateConcurrencyException as progress instead of an error-level log.

Verification

• Full solution build: clean, 0 warnings/errors.
• Touched test projects, both target frameworks: all green (Inbox core, Inbox.Relational,
  SharedKernel.Outbox), plus downstream Outbox.EntityFrameworkCore and Messaging.Outbox suites
  re-run as a sanity check.
• dotnet pack -c Release for every touched src project: APICompat reports no breaking changes
  against the 1.1.0 baseline for all four packages.
• PublicAPI files are unchanged except the one documented optional-parameter text edit.

Backport to v1.0: no — internal dedup and tests ship with the next release.