fix(pricing): add MiniMax catalog entries

[octo-patch]

Reason: The pricing catalog missed MiniMax M3 and M2.7, so those models fell through to unknown-model pricing.

Changes:

• Added curated price rows for MiniMax M3 and M2.7 in src/session/model-prices.json.
• Added lookup and cost regression coverage in tests/session/pricing.test.ts.

Checks:

• pnpm exec vitest run tests/session/pricing.test.ts
• pnpm exec tsc --noEmit
• git diff --check
• secret scan: no matches