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

[github-actions]

Implements the Phase 2 test coverage improvements planned in #55.

What's added

src/lib/api/validation-utils.test.ts — 22 new tests

Covers validateObject and validateRequiredString with table-driven it.each cases:

• Returns error message for null, undefined, numbers, strings, booleans
• Returns null for valid objects (including {})
• Error messages include the fieldName parameter
• validateRequiredString rejects empty strings, whitespace-only strings, and non-string values

src/lib/api/response-utils.test.ts — 17 new tests

Covers apiSuccess, validationErrorResponse, and serviceUnavailableResponse:

• Correct HTTP status codes (200 / 400 / 503)
• Response body shape: success, data/error, optional meta
• Handles various data types (objects, arrays, null)
• meta is omitted when not provided, included when passed

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

Covers getNextAchievableSkills with realistic scenarios:

• Empty profile returns all foundation skills (no prerequisites)
• Skills with unmet prerequisites are excluded
• Skills are unlocked when prerequisites are present at any level
• Skills already at intermediate/advanced are excluded
• Skills at beginner remain achievable (not yet intermediate)
• Multi-prerequisite skills unlock only when all prerequisites are present
• Cascading prerequisites (nextjs → react+typescript)
• Duplicate skill entries use the highest level
• Returned nodes have correct shape

Coverage comparison

Metric	Before	After
Test files	63	66
Passing tests	1025	1077
New tests	—	+52

Pre-existing failures in src/lib/github/client.test.ts (10 tests, missing token in CI) are unchanged — as noted in #55.

All new tests pass. TypeScript (npx tsc --noEmit) is clean.

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