feat(webhooks): add signing secret rotation endpoint

[gabrielmfern]

Adds resend.Webhooks.rotate_signing_secret(webhook_id) and its async twin for POST /webhooks/{webhook_id}/signing-secret/rotate, returning RotateSigningSecretResponse with the same object, id, signing_secret shape as create webhook. Mirrors the event replay endpoint from #274 in layout, tests, and the examples/webhooks.py walkthrough.

Spec: resend/resend-openapi#113
Linear: https://linear.app/resend/issue/DEV-2072

🤖 Generated with Claude Code

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Summary by cubic

Adds a signing secret rotation endpoint so users can rotate a webhook's signing secret without recreating it.

• Adds resend.Webhooks.rotate_signing_secret(webhook_id) and the async twin rotate_signing_secret_async.
• Returns a new RotateSigningSecretResponse with the same object, id, and signing_secret shape as the create webhook response.
• Mirrors the existing event replay endpoint in structure, tests, and the examples/webhooks.py walkthrough.
• Bumps the package version to 2.44.0.

^{Written for commit 99cb3b0. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 4 files

<sub>Reply with feedback, questions, or to request a fix.

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[cubic-dev-ai]

0 issues found across 1 file (changes from recent commits).

Confidence score: 5/5

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

<sub>Requires human review: Auto-approval blocked by 2 unresolved issues from previous reviews.

Re-trigger cubic</sub>