Add JSDoc to exported helpers in src/utils/searchUtils.ts

[nanaf6203-bit]

Context

src/utils/searchUtils.ts exports filter-parsing/serialization helpers used by search UI, but exported functions lack JSDoc - in tension with the repo's jsdoc/require-jsdoc policy and with src/utils/secureId.ts-style documented utilities.

Problem

Contributors cannot tell parameter semantics (e.g., how locale affects URL keys) without reading implementations, and lint may flag new files inconsistently.

Proposed approach

Add concise JSDoc (purpose, params, returns, example) to each export; ensure npm run lint passes for the file.

Acceptance criteria

• All exports documented
• Lint clean for the file
• Docs mention key locale behavior