test: cover an untested search preference edge case

[TuYv]

Goal

Add one focused regression test for an existing search or preference edge case that is not already covered.

Acceptance criteria

• Identify the uncovered behavior in a comment on this issue before implementation.
• Add the test under tests/ using node:test and node:assert/strict.
• Do not change production behavior unless the test exposes a confirmed bug.
• node --test tests/*.test.js passes.

Contributor notes

Please keep the PR focused on one behavior and link this issue.

[YRWoods]

Uncovered behavior I'll test (one focused edge):

normalizeSearchPreferences currently has tests for missing/undefined, explicit booleans, non-boolean values, and resultsLimit allow-list — but not for non-object payloads (null, string, number, array). Those should fall back to DEFAULT_SEARCH_PREFERENCES without throwing, same as an empty object.

Also assert unknown keys are stripped (normalized output only has preference keys).

No production change planned unless the test fails for a real bug.

[YRWoods]

Re-opened as PR #23 (same focused non-object payload coverage for #11).