fix(api): add spacing around human input form urls

[Theysua]

When {{#url#}} is adjacent to non-whitespace text in a Human Input email delivery body, the rendered URL was concatenated directly with surrounding characters, producing broken links (e.g. "Approval:https://...").

Fix by replacing the simple str.replace call with a regex substitution that inspects the characters immediately before and after the placeholder and inserts a space where none exists.

Fixes #34160

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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint and make type-check (backend) and cd web && npx lint-staged (frontend) to appease the lint gods