[FEATURE]: Support Astra async tools and mid-turn steering

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Describe the enhancement you want to request

OpenCode v2 should support GPT-6 Astra's Responses API features end to end: async function tools and mid-turn steering.

For async tools, OpenCode should mark eligible local functions with async: true, execute them concurrently with model output, preserve the original call_id, and deliver completed results in a later continuation. For steering, a plain-text inbox update should be persisted before response.steer is sent, then handle accepted, failed, pending, normally completed, and automatically continued responses without duplicating user input.

This keeps Astra sessions responsive during long tools and lets users redirect active work without losing durable session history. The implementation should retain HTTP fallback, permission handling, recovery after disconnects, and non-retry behavior for misalignment_policy_violation.

Implementation draft: #47536