feat: add Emails.share method

[dielduarte]

Adds Resend::Emails.share(email_id, params = {}), calling the new POST /emails/{id}/share endpoint that creates a shareable link for a sent or received email. See the spec PR resend/resend-openapi#92.

params[:expires_in] is an optional human-readable duration (e.g. 10m, 2 hours, 1 day; server defaults to 48h, capped at 48h).

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Summary by cubic

Add Resend::Emails.share to create a shareable link for a sent or received email via POST /emails/{id}/share. Previously you could not generate share links; now you can optionally set expires_in with a human-readable duration. The server defaults to 48h and caps at 48h.

• Posts to emails/{id}/share; includes a body only when expires_in is provided.
• Returns id and url. Maps validation and not found responses to existing Resend::Error classes.
• Specs cover default expiry, custom expires_in ("10m"), invalid durations (>48h), and missing email (404).
• No breaking changes or configuration.

^{Written for commit a5f2467. 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 Resend::Emails.share to create a shareable email link via POST /emails/{id}/share. It's a minimal, well-tested, non-breaking addition with no auth/config/security impact.

Re-trigger cubic</sub>

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