Fix warning Email alert

[bmfmancini]

Fixes #750

[Copilot]

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[somethingwithproof]

Reviewed again: the three warning threshold_value substitutions are internally consistent and my approval stands. Collision note: this edits the type-2 notification block in thold_functions.php, adjacent to the threshold extraction work in #801. Merge this small fix first, then rebase #801 and the other threshold PRs; do not copy these same three hunks into every issue branch.

[somethingwithproof]

CI audit: this PR currently has an empty diff against develop, so there is no remaining plugin change to validate. Its only non-passing check is the obsolete Cacti develop advisory matrix job, which has been stuck in apt-get update for hours. This PR appears obsolete and should be closed rather than changing its head into a CI-only branch.