Map current duplicate-start conflicts to WorkflowAlreadyStarted #164

Confirmed published-client defect

During #163 qualification, normally installed Python SDK 2.6.0 against immutable PHP Server 2.9.5 (sha256:f5771101fc55a3e1e8f8bd572beb1d57f593c05db5e5760f0773f75a96fe7ebb) raises generic ServerError for a duplicate start. The actual response is HTTP 409 / outcome=rejected_duplicate / reason=instance_already_started. errors.py maps only the historical duplicate_not_allowed reason to WorkflowAlreadyStarted.

Three independent originals (initial policy omitted, fail, use-existing) all reproduce this at the first active duplicate. The later policy/completion assertions were not reached. Three separate invalid-policy fresh-ID controls already pass with 422 and no admitted workflow. No replacement, termination or PHP defect is inferred.

Acceptance:

• Map the current duplicate-start reason to the existing typed exception, retaining the historical reason.
• Preserve unrelated conflicts and exact workflow identity.
• Add focused current/legacy mapping coverage and qualify active/closed duplicate behavior with published PHP Server using the unchanged lifecycle and wire policies.
• Deliver a compatible patch and verify normal installation. Do not bundle unrelated unreleased behavior into the patch.

General owns this alongside #163. This is an exception-classification correction, not a workflow reuse or termination feature. Native parity remains unqualified.