I18N: Add missing translator comments and fix string issues for 7.0

[sanketio]

Summary

This PR addresses several I18N string improvements as part of the WordPress 7.0 release cycle.

Changes

• Added missing translator comments to several user-facing error messages that lacked context for translators. These include SMTP-related error strings and informational messages containing technical terms (PHP function names, protocol extension names, and email standard names) that translators need to know should not be translated.
• Fixed a misplaced translator comment that was positioned too far from its associated translatable string (separated by a conditional block), causing translation tooling such as WP-CLI i18n make-pot and Poedit to fail to associate the comment with the string.
• Fixed inconsistent capitalization in a user-facing warning string where the first word after the label prefix was lowercase, inconsistent with WordPress core style.

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Trac ticket

• https://core.trac.wordpress.org/ticket/64244

Use of AI Tools

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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[mukeshpanchal27]

For string what is benefit of having translators comment here? Better to use that when use placeholders (e.g., %s, %d).