fix(smtp): normalize error messages to Go convention

[Copilot]

Error messages in smtp/smtp.go violated Go conventions by using capitalized prefixes ("SMTP host required", "SMTP from address required"). Go errors must be lowercase with no trailing period.

Changes

• smtp/smtp.go: Lowercased two non-conforming error strings to match the style of existing errors in the same function:

  // Before
  fmt.Errorf("SMTP host required")
  fmt.Errorf("SMTP from address required")
  
  // After
  fmt.Errorf("host required")
  fmt.Errorf("from address required")

[Copilot]

Pull request overview

This PR adjusts SMTP configuration validation errors to better align with Go’s error-string conventions (lowercase, no trailing period) within the smtp package.

Changes:

• Lowercased the “host required” validation error returned when Config.Host is empty.
• Lowercased the “from address required” validation error returned when Config.From is empty.

Show a summary per file

File	Description
smtp/smtp.go	Normalizes two validation error strings returned by Config.Validate().

Copilot's findings

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• Files reviewed: 1/1 changed files
• Comments generated: 2

[Copilot]

PR description claims SMTP error messages are normalized to Go conventions, but this file still returns several capitalized error prefixes (e.g., "SMTP connection failed", "SMTP client failed", "SMTP auth failed"). Either update those as well or narrow the PR description/scope so it matches the actual changes.