fix(server): a busy provider switch fails with its own tagged error

The active-turn guard stored its reason as a synthetic cause on a generic
dispatch error. It now raises OrchestratorProviderSelectionBusyError, whose
message comes from its attributes, and intake treats it as not accepted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
6b3bdce4a6Julius Marminge committed on 10/22/2026, 8:31:23 AMparent896c0e1

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