refactor(otel): centralize command span tagging via ITraceable interface

[falberthen]

Added

Introduces ITraceable, a dedicated interface in Core.Infrastructure.OpenTelemetry, to centralize span tagging in the CommandBus instead of scattering Activity.Current?.SetTag(...) calls across command handlers.

• Commands that carry a known OrderId implement ITraceable.GetSpanTags(). The CommandBus applies the tags automatically when dispatching, only when an active span exists.
• Handlers where the tag value is not known at dispatch time (e.g., PlaceOrderHandler, where the order ID is generated inside the handler) retain the direct SetTag call as a documented exception.
• Keeps ICommand as a pure dispatch contract and isolates the observability concern behind its own interface.

Removed

Removes Activity.Current?.SetTag("order.id", ...) from command handlers. The tag is now applied centrally by CommandBus for any command implementing ITraceable, eliminating the risk of new handlers silently missing the annotation.