fix: lazy import resend in email_tool

[Hundao]

Description

Move import resend from module level to inside _send_via_resend() with a try/except guard. Without this, missing resend package crashes all tool registration.

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Related Issues

Fixes #4816

Changes Made

• Removed bare import resend at module level
• Added lazy import with try/except inside _send_via_resend(), returns error dict when not installed

Testing

• Lint passes (cd core && ruff check .)
• Manual testing performed

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code