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

[github-actions]

Summary

Phase 2 of the Daily Test Coverage Improver plan. Implements the top 3 priorities from the research issue.

New Test Files

src/lib/api/validation-utils.test.ts — 28 tests

Tests for validateObject and validateRequiredString:

• Valid objects (empty, with properties, arrays, nested)
• Invalid values: null, undefined, numbers, strings, booleans
• Custom fieldName appears in error messages
• Whitespace-only strings rejected by validateRequiredString

src/lib/api/response-utils.test.ts — 11 tests

Tests for apiSuccess, validationErrorResponse, serviceUnavailableResponse:

• Correct HTTP status codes (200, 400, 503)
• success: true/false field set correctly
• meta included only when provided
• Uses vi.mock('next/server') to isolate from Next.js runtime

src/lib/skills/prerequisites.test.ts — 13 tests

Tests for getNextAchievableSkills (complex pure function):

• Empty profile → only foundation skills (javascript, python, html, sql, git)
• Prerequisites unlock skills at any level (even beginner)
• Multi-prerequisite skills (react, ci-cd) require ALL dependencies
• Skills at intermediate/advanced level are excluded from results
• Duplicate skill entries use the highest level
• SKILL_PREREQUISITES data integrity: unique IDs, no unknown prerequisite references

Coverage Improvement

File	Before	After
src/lib/api/validation-utils.ts	0%	~100%
src/lib/api/response-utils.ts	0%	~100%
src/lib/skills/prerequisites.ts	0%	~100%

Test Results

Test Files  3 passed (3)
     Tests  52 passed (52)

Full suite: 1077 passing (up from ~1041), 10 pre-existing failures in client.test.ts unchanged (token mocking issue tracked separately).

Notes

• All tests follow project conventions: describe/it blocks, it.each for table-driven cases, AAA pattern
• Tests are colocated with source files per project conventions
• Pre-existing client.test.ts failures are unrelated to these changes (missing gh auth in CI)

Generated by Daily Test Coverage Improver

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