Fix critical type safety and validation issues

Issue 2 - Type Safety:
- Remove dangerous 'as any' casts in sendEmail function
- Use proper typing for payload.create() calls
- Maintain type safety throughout email creation process

Issue 3 - Email Validation:
- Implement RFC 5322 compliant email regex
- Add comprehensive validation for common invalid patterns
- Check for consecutive dots, invalid domain formats
- Prevent emails like 'test@.com' and 'test@domain.'

Issue 4 - Error Message Logic:
- Add contextual error messages for template vs direct email modes
- Distinguish between template rendering failures and missing direct email content
- Provide clearer guidance to developers on what went wrong

Additional fixes:
- Update imports to use generated Email type instead of BaseEmailData
- Maintain compatibility with updated sendEmail interface

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/index.ts

@@ -16,7 +16,7 @@ export { mailingJobs, sendEmailJob } from './jobs/index.js'
 export type { SendEmailTaskInput } from './jobs/sendEmailTask.js'
 
 // Main email sending function
-export { sendEmail, type BaseEmailData, type SendEmailOptions } from './sendEmail.js'
+export { sendEmail, type SendEmailOptions } from './sendEmail.js'
 export { default as sendEmailDefault } from './sendEmail.js'
 
 // Utility functions for developers