Verify and fill in real Vertex prices in prices.json

[yuting0624]

prices.json ships with placeholder/estimated Vertex rates, and every cost figure depends on it.

Task: confirm the current Vertex AI rates (Opus 4.8, Sonnet 4.6, Gemini 3.5 Flash, Gemini 3.1 Pro) and update prices.json. Cite the pricing page in the PR.

Acceptance: values match a linked official source; bash tests/run-tests.sh still passes.

Good first issue — no deep code, high impact.