docs(gemini): document canonical usage cost

[acedatacloud-dev]

Summary

• document Gemini native per-call pricing at canonical usage.cost
• preserve native usageMetadata token fields
• add real response-derived non-streaming and terminal SSE examples
• clarify best-effort cost preview and native stream termination semantics

Verification

• git diff --check — passed
• examples use sanitized values from live generateContent and streamGenerateContent responses

Release order

Publish after the companion PlatformService runtime PR is deployed.

🤖 对抗评审 (reviewer: codex, 2 rounds)

• RISK: companion guide advertised audio without verified modality pricing → fixed by reverting that unrelated capability change
• Round 2: VERDICT: CLEAN

🤖 Generated with Claude Code