chore(internal): helping ecosystem-ci

[jycouet]

Description

Test fixtures in tailwindcss and mdsvex addon tests appended markup without checking for prior insertion. With retry: 3 in CI, a flaky first attempt would compound: each retry duplicated the fixture nodes, turning recoverable flakes into hard strict mode violation failures (e.g. this run).

Both fixtures now use transforms.svelte / transforms.svelteScript from sv-utils and short-circuit via return false when a marker node is already present (same pattern as addToDemoPage in addons/common.ts).

Checklist

• Update snapshots (if applicable)
• Add a changeset (if applicable) - n/a, internal test infra only
• Allow maintainers to edit this PR
• I care about what I'm doing, no matter the tool I use (Notepad, Sublime, VSCode, AI...)

[changeset-bot]

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Latest commit: 6ec4576

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commit: 6ec4576