Fix spawn! options-wrapping bug on tool-call rounds 2+

[DeanSingh]

Summary

• spawn! passed options: @options which wrapped provider options one level deep on each recursive tool-call round. Round 2+ received { options: { thinking: true } } instead of { thinking: true }, silently dropping thinking config and any other provider-specific options.
• Fixed to **@options so the hash is splatted back into keyword arguments correctly.
• Added test covering options propagation through multi-round tool-call sessions.

Impact

All providers (Google, Anthropic, OpenAI) inherit spawn! from the base Chat class. Any provider-specific option passed via **options (e.g. thinking:, effort:) was lost on rounds 2+. This is the root cause of thinking deltas only appearing on round 1 in multi-round tool-calling sessions.

Test plan

• bundle exec rspec passes (437 examples, 0 failures)
• bundle exec rubocop passes (0 offenses)
• New test verifies spawn! preserves options flat across rounds
• New test verifies multi-round tool-call session completes with options
• Verified live against Vertex AI with gemini-3.1-pro-preview: thinking deltas received on rounds 2+