Extract build*Form() methods from each huhPrompter method, separating
form construction from form.Run(). This enables testing the real form
construction code by driving it with direct model updates, adapted
from huh's own test patterns.
Tests cover Input, Select, MultiSelect, Confirm, Password,
MarkdownEditor, and MultiSelectWithSearch including a persistence
test that verifies selections survive across search query changes.
Also fixes a search cache initialization bug where the first
buildOptions("") call would skip the searchFunc due to
cachedSearchQuery defaulting to "".
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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