Add comprehensive tests for EnhancedEmailManager covering:
- Rate limiting (exceeds limit, resets after minute, concurrency, negative values)
- Provider username defaults (SendGrid, SparkPost, Resend, Postmark)
- TLS routing (TLS, StartTLS, plain connections)
- Retry logic (retries on failure, rate limit prevents retry)
- Connection error handling for sendTLS, sendStartTLS, sendPlain
This is the first test file for email_enhanced.go which previously had
0% coverage on sendTLS, sendStartTLS, and TestConnection functions.
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