Address copilot-pull-request-reviewer feedback on #2157:
- billing_service_test.go: extend TestGetFallbackPricing_FamilyMatching
with optional expectedOutput / expectedCacheRead fields and assert
full Input/Output/CacheRead pricing for all 4 DeepSeek cases
(v4-pro, v4-flash, deepseek-chat→flash, deepseek-reasoner→flash),
preventing silent regression to 0 output/cache cost.
- billing_service.go: rewrite the DeepSeek block comment to explicitly
describe its scope (V4 Pro/Flash + chat/reasoner aliases, no
unknown-deepseek fallback) and tighten the OpenAI comment to make
it unambiguous that it only describes the OpenAI/Codex branch
immediately below it.