test: add unit tests for API utils and skill prerequisites (Phase 2)

[github-actions]

Closes #55

Summary

Phase 2 implementation from the Daily Test Coverage Improver plan. Adds 62 new passing tests across 3 previously untested files.

New Test Files

src/lib/api/validation-utils.test.ts — 21 tests

Tests validateObject and validateRequiredString pure functions:

• null, undefined, wrong-type inputs → error message returned
• valid object / non-empty string → null returned
• fieldName interpolated correctly in error messages
• Whitespace-only strings rejected, single-char strings accepted
• Table-driven with it.each

src/lib/api/response-utils.test.ts — 21 tests

Tests apiSuccess, validationErrorResponse, serviceUnavailableResponse:

• Correct HTTP status codes (200, 400, 503)
• success: true/false in body
• Data and meta fields present/absent as expected
• Error messages preserved verbatim
• Various data shapes (object, array, null, string)

src/lib/skills/prerequisites.test.ts — 20 tests

Tests getNextAchievableSkills and SKILL_PREREQUISITES:

• Empty profile → all foundation skills returned
• Skill at intermediate/advanced → excluded from results
• Skill at beginner → still returned as "next achievable"
• Prerequisite chains: typescript unlocked by JS, react by JS+HTML+CSS, nextjs by react+TS, etc.
• Partial prerequisites → skill NOT unlocked
• Duplicate skill entries → highest level wins
• Data integrity: all prerequisite IDs reference valid skill nodes

Coverage Comparison

Metric	Before	After
Test files	63	66
Tests passing	1025	1087
src/lib/api/validation-utils.ts	0%	~100%
src/lib/api/response-utils.ts	0%	~100%
src/lib/skills/prerequisites.ts	0%	~100%

Pre-existing failures in src/lib/github/client.test.ts (10 tests) are unchanged — these require a separate fix.

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