test: expand coverage to src/services/ (snapshot-builder, token-optimizer, ai-client)

[hoainho]

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What

vitest.config.ts line 13 restricts coverage to src/panel/**. The most important pure-logic modules in the codebase are excluded from coverage and have zero tests:

File	Public API	Test coverage
src/services/snapshot-builder.ts	buildSnapshot, hashSnapshot, snapshotToPromptText	0
src/services/token-optimizer.ts	TokenOptimizer class (5 methods)	0
src/services/ai-client.ts	analyzeSnapshot, response parsing	0
src/utils/messaging.ts	6 utility functions	covered by 1 test (messaging.test.ts)
src/utils/sanitize.ts	2 exported functions	covered

Why it matters

• These services are pure logic — easy to test, no Chrome mocking needed
• hashSnapshot uses crypto.subtle — a regression could break deduplication silently
• checkRateLimit is a sliding window — off-by-one would let users exceed quotas
• AI client error paths are completely untested

What to do

This is a help wanted issue because it's M-sized (~6h). Suitable for someone who:

• Wants a bigger contribution than a 1-line fix
• Likes writing tests
• Has time to learn the codebase's testing conventions (src/__tests__/setup.ts)

Suggested deliverables

1. src/__tests__/snapshot-builder.test.ts with ≥6 tests
2. src/__tests__/token-optimizer.test.ts with ≥6 tests
3. (Optional) src/__tests__/ai-client.test.ts with happy-path + error tests using fetch mocks
4. Update vitest.config.ts to include src/services/** and src/utils/** in coverage

Test ideas — snapshot-builder

• buildSnapshot with empty TabState → all arrays empty, memory: null
• buildSnapshot with 51 issues → top 50 retained (MAX_ISSUES enforcement)
• snapshotToPromptText with full snapshot → contains section headers
• hashSnapshot deterministic: same input → same hash
• hashSnapshot different snapshots → different hashes

Test ideas — token-optimizer

• checkRateLimit with 0 calls → allowed: true, remainingCalls: 3
• After 3 recordCall() → allowed: false
• getCachedResult with expired entry → returns null
• setCachedResult at MAX_CACHE_ENTRIES → evicts oldest
• validateSubscriptionKey with valid/invalid format

Acceptance criteria

• At least 2 new test files committed
• All new tests pass
• vitest.config.ts coverage include widened to services + utils
• npm run test:coverage shows ≥60% coverage of src/services/
• No changes to production code (tests should not require refactoring)

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Scope: M (3 new test files + config) | Effort: ~4-6h | Risk: Lane Tiny | Label: help wanted