feat(automations): duplicate endpoint

[gabrielmfern]

Adds resend.automations.duplicate(automation_id) for POST /automations/{id}/duplicate, returning DuplicateAutomationResponse.

Resolves DEV-1710. Mirrors the existing stop pattern and resend-node resend/resend-node#1071.

🤖 Generated with Claude Code

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Summary by cubic

Adds resend.automations.duplicate(automation_id) to duplicate an automation via POST /automations/{id}/duplicate. Previously the Rust SDK lacked duplication; now it returns DuplicateAutomationResponse with the new automation id. Supports DEV-1710 parity with other SDKs.

• Implements AutomationsSvc.duplicate following the existing stop pattern and maybe_async.
• Introduces types::DuplicateAutomationResponse and re-exports it from src/lib.rs.
• Adds a test for duplicate→delete; no breaking changes or migrations required.

^{Written for commit 7f50b06. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 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 SDK method and response type following the existing stop pattern, with a test exercising the new endpoint. Additive change with no behavior, config, or contract modifications requiring human judgment.

Re-trigger cubic</sub>

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