fix: return undefined from getPrefix/parseId for non-string input

[shaurya703]

getPrefix() and parseId() crashed with TypeError: Cannot read properties of null (reading 'indexOf') when handed non-string input, while isId() and getTimestamp() already tolerated it. This made half the read-side API unsafe for untrusted values from request bodies, query strings, and databases — especially in plain JavaScript where there is no compiler to catch the mismatch.

Changes

• Added a typeof value !== "string" guard returning undefined at the top of both getPrefix and parseId in src/utils/validate.ts, matching the existing isId / getTimestamp behaviour.
• Added tests to test/validate.test.ts covering null, undefined, numbers, and objects for both functions.
• CHANGELOG entry under Unreleased.

Verification

npm run format, npm run typecheck, npm test (108 passing), npm run build, and npm run smoke all pass.

Fixes #10