Fix TokenRouter GPT-5 OpenCode tool calls

[ShuxinLin]

Summary

• disable OpenCode reasoning_effort for TokenRouter-hosted OpenAI GPT-5 models so function/MCP tools work on chat/completions
• surface OpenCode error events as RuntimeError instead of returning an empty trajectory/answer
• add coverage for tokenrouter/openai/gpt-5.5 and tokenrouter/openai/gpt-5.6-sol

Closes #443

Verification

• uv run pytest src/agent/opencode_agent/tests/test_runner.py -q
• uv run opencode-agent --model-id tokenrouter/openai/gpt-5.6-sol --show-trajectory --max-steps 3 "hello"
• uv run opencode-agent --model-id tokenrouter/openai/gpt-5.5 --show-trajectory "hello"