feat(automations): duplicate endpoint

[gabrielmfern]

Adds Automations.Duplicate(automationId) and Automations.DuplicateWithContext(ctx, automationId) for POST /automations/{id}/duplicate.

Resolves DEV-1709. Mirrors the existing Stop pattern and resend-node resend/resend-node#1071.

🤖 Generated with Claude Code

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Summary by cubic

Adds Automation duplicate support to the Go SDK so clients can duplicate an automation by ID. Previously unsupported; now Automations.Duplicate(...) and Automations.DuplicateWithContext(...) wrap POST /automations/{id}/duplicate to satisfy DEV-1709 and maintain parity with resend-node.

Changes and review notes

• Add DuplicateAutomationResponse and POST path automations/{id}/duplicate.
• Extend AutomationsSvc and AutomationsSvcImpl with Duplicate/DuplicateWithContext.
• Add ErrFailedToCreateAutomationDuplicateRequest.
• Add TestDuplicateAutomation.
• Minor struct field alignment; no behavior change.

Migration

• If you implement AutomationsSvc (e.g., custom mocks), add Duplicate and DuplicateWithContext to your implementation.

^{Written for commit 969cb34. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Auto-approved: Adds a new Duplicate endpoint mirroring the existing Stop pattern, with a focused test and no changes to existing behavior or contracts.

Re-trigger cubic</sub>

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