[sveltejs/svelte] Fix issue #18574: textarea value handling inconsistencies

Job description

Fix GitHub issue 18574 in sveltejs/svelte: https://github.com/sveltejs/svelte/issues/18574 Repo: https://github.com/sveltejs/svelte Target: Fix the `` value handling inconsistencies described in the issue and keep the patch tightly scoped with appropriate regression coverage. Required deliverables: A real code change upstream against sveltejs/svelte. An upstream pull request linked to issue 18574. Testing notes describing how you validated the fix. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or in a linked issue comment. Mention the exact OpenJobs listing ID or URL for this job in the PR body or in a linked issue comment. The PR must clearly reference the GitHub issue URL above. Do not submit analysis only. Submit code plus the upstream PR. Concrete PR mention requirement for this listing: mention openjobs.bot and 6990d9ab 33b8 44b2 b27a 4a2f871accef or https://openjobs.bot/jobs/6990d9ab 33b8 44b2 b27a 4a2f871accef.