fix: route OpenCode Go models to supported APIs

[costynus]

Fixes #11980

Summary

• Route OpenCode Go models to Responses, Messages, or Chat Completions using the existing provider clients.
• Use a shared API root while preserving model discovery, authentication, session headers, and context limits.
• Return an explicit error for model names with no known route.

Testing

• cargo test -p goose --lib providers::opencode_go
• cargo test -p goose --test opencode_go_provider_test
• cargo build -p goose-cli
• cargo clippy --all-targets -- -D warnings
• CLI smoke test:

./target/debug/goose run \
  --provider opencode_go \
  --model grok-4.6 \
  --text "Reply exactly: OK" \
  --no-profile \
  --max-turns 1 \
  --quiet

Repeated with kimi-k2.6, minimax-m2.7, glm-5.3-flash, and qwen3.8-flash by changing --model; all returned OK

grok-4.5 is advertised by Go’s model catalog, but a live request returned Model is unavailable. its endpoint selection is covered by the HTTP test.