feat(client): add update-with-start support

[discosultan]

Closes #1136

Adds atomic update-with-start to the client:

• Client::start_update_with_start_workflow and Client::execute_update_with_start_workflow, backed by the ExecuteMultiOperation RPC, with WithStartWorkflowOperation describing the start request.
• Multi-operation failures are decoded and attributed to the failing operation (WorkflowUpdateWithStartError::Start/Update).
• Interceptor support via ClientInterceptor::update_with_start_workflow.

Tested against local dev server.

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