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

[github-actions]

Implements the Phase 2 test coverage plan from #55.

What's added

src/lib/api/validation-utils.test.ts (22 tests)

Covers validateObject and validateRequiredString:

• Invalid inputs: null, undefined, non-string types, non-object types
• Whitespace-only strings correctly rejected
• Valid objects and non-empty strings return null
• Uses it.each for table-driven test cases

src/lib/api/response-utils.test.ts (15 tests)

Covers apiSuccess, validationErrorResponse, serviceUnavailableResponse:

• Correct HTTP status codes: 200 / 400 / 503
• success: true/false field on all responses
• meta included when provided, omitted when not
• Various data types handled correctly by apiSuccess

src/lib/skills/prerequisites.test.ts (11 tests)

Covers getNextAchievableSkills:

• Empty profile returns all foundation skills
• Skills with unmet prerequisites are excluded
• Skills at beginner remain achievable, intermediate/advanced are excluded
• Cascading prerequisites (e.g. nextjs requires react + typescript)
• Duplicate skill entries: highest level wins
• All skills intermediate+ → empty result

Coverage comparison

Metric	Before	After
Test files	63 (3 failing pre-existing)	66 (same 3 failing)
Tests	1035 (10 failing)	1083 (same 10 failing)
New tests added	—	+48

The 10 pre-existing failures are all in src/lib/github/client.test.ts (token unavailable in CI) — unchanged.

Closes #55

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