[FEATURE] Honor provider-reported cost (usage.cost) from LLM gateways (OpenRouter, LiteLLM, Manifest)

Problem

opencode computes message cost client-side: token counts × models.dev catalog rates in getUsage (packages/opencode/src/session/session.ts). The only provider-reported cost honored today is the hardcoded Copilot providerMetadata.copilot.totalNanoAiu special case.

LLM gateways/routers report the actual USD cost per request in the response usage block, and opencode ignores it:

• OpenRouter: usage.cost + usage.cost_details (with usage: {include: true})
• LiteLLM: include_cost_in_streaming_usage → usage.cost
• Manifest (mnfst/manifest): stamps usage.cost on proxied OpenAI/Anthropic responses, streaming and non-streaming

For router models (openrouter/auto, manifest/auto, DigitalOcean Inference Routers) local catalog rates can never price the request correctly — the routed upstream model varies per request, so cost shows $0.00 or a wrong amount. Related: #39869, #34877, #40835.

Proposal

1. Capture usage.cost into step providerMetadata. @ai-sdk/openai-compatible parses usage into its own schema and drops unknown keys like cost, so this needs either a custom fetch wrapper in opencode's provider layer (same pattern as existing per-provider fetch customizations) or an upstream AI SDK change to forward extra usage fields.
2. Consume it in getUsage: generalize the Copilot branch — when provider metadata carries a reported USD cost, prefer it over catalog rates; fall back to local computation otherwise.
3. No UI changes needed — assistantMessage.cost is already rendered in the TUI; it just needs the right value.

Notes

• Works for streaming (final usage chunk) and non-streaming responses.
• A reported cost of 0 is meaningful (flat-fee subscription routed request) and should still win over catalog rates.
• Fixes cost display for OpenRouter users today, not just new gateways.