feat(automations): duplicate endpoint

[gabrielmfern]

Adds AutomationDuplicateAsync(automationId) for POST /automations/{id}/duplicate, returning the new automation identifier.

Resolves DEV-1712. Mirrors the existing AutomationStopAsync/AutomationCreateAsync patterns and resend-node resend/resend-node#1071.

🤖 Generated with Claude Code

---

Summary by cubic

Adds automation duplication to the .NET SDK so clients can clone existing automations. Previously duplication wasn’t available; now AutomationDuplicateAsync(Guid) posts to /automations/{id}/duplicate and returns the new automation ID. Supports DEV-1712.

• Adds AutomationDuplicateAsync(Guid, CancellationToken) to IResend and implements it in ResendClient, mapping ObjectId.Id to Guid.
• Adds test AutomationDuplicate and a local API stub for automations/{id}/duplicate.
• Bumps fallback version in Directory.Build.props to 0.10.0.
• No breaking changes; existing automations APIs are unaffected.

^{Written for commit 101da2e. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 4 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 that mirrors existing automation patterns, with a test and local API stub. No schema, auth, or operational changes.

Re-trigger cubic</sub>

Approval not submitted

This repository is configured to approve as @klotty, but that approval identity is unavailable.
The refresh token passed is incorrect or expired.

cubic left this as a normal review comment instead of falling back to a Cubic App approval.

[gabrielmfern]

@cursor can you bump the fallback version here to 0.10.0?

[cursor]

Done — I bumped the fallback version in Directory.Build.props from 0.7.0 to 0.10.0 and pushed it to PR #137.

That <Version> property is the fallback used when the release pipeline doesn't override it: cicd/release.sh passes -p:Version= from the git tag on releases, so local and CI builds fall back to the props value, which was stale at 0.7.0 (latest tag is v0.9.0). I verified with a Release build that the package now produces Resend.0.10.0.nupkg, and left the ?? "0.0.0" User-Agent guard in ResendClient.cs untouched since that's an "unknown version" placeholder rather than the release fallback.