Follow-up to #3764.

Summary

• Extract the inline findSuggestionTemplatePlaceholder() function and SUGGESTION_TEMPLATE_PLACEHOLDER_PATTERN constant from input-box.tsx into a standalone core/suggestions/placeholders.ts module
• Add 10 unit tests for the extracted utility (8 for findSuggestionTemplatePlaceholder range detection + 2 for hasUnreplacedPlaceholder boolean wrapper)
• Replace SuggestionList's raw textareaRef prop with an onSelectPlaceholder callback for better component decoupling
• Change toast.error to toast.warning for placeholder validation (amber vs red — it's a validation hint, not an error)

Why

#3764 introduced an inline findSuggestionTemplatePlaceholder at the top of input-box.tsx. This works fine, but:

1. Not testable in isolation — the function lives inside a 1300-line component file. Extracting it to core/suggestions/ makes it independently unit-testable and reusable.
2. SuggestionList receives a raw textareaRef — this couples the child to the parent's DOM ref type and leaks implementation details. A callback prop (onSelectPlaceholder) inverts control properly: the child signals intent, the parent decides how.
3. toast.error overstates severity — an unfilled placeholder is a validation hint (user needs to act), not a system error. toast.warning communicates this better.

Changes

Test Plan

• 10 unit tests for the extracted utility — all pass
• tsc --noEmit — no errors
• eslint — no new errors
• Existing E2E test from fix: block unresolved suggestion template placeholders #3764 continues to work (no behavioral changes)