feat(acp): add unstable v2 session injection

[danielkov]

Summary

Adds ACP v2 session injection to AgentKit behind the unstable-inject feature.

• accepts steer injections while a turn is running
• lets the current model stream or tool call finish before delivery
• delivers pending messages FIFO before the next model request
• emits each delivered message as a v2 user_message update
• supports revoke and retains delivered IDs for correct error responses
• keeps accepted messages across cancel and discards them on session close

The same injection coordinator is used by AcpHeadlessRuntime and by hosts with custom persistent session actors. The public host API covers session binding, update delivery, prompt conversion, turn lifecycle, cancellation, revoke, and safe-boundary delivery without exposing the internal locks or queues.

Dependencies

• ACP v2 foundation: feat(acp): add opt-in ACP v2 runtime foundation #12
• Schema: feat(unstable-v2): add session injection schema agentclientprotocol/agent-client-protocol#2043
• Rust SDK response receipts: feat(jsonrpc): add tracked response receipts agentclientprotocol/rust-sdk#338
• Rust SDK injection support: feat(unstable-v2): add session injection support agentclientprotocol/rust-sdk#339

The branch currently pins schema 6e7e044f9464c4fd652d90699a09e9edc8b3bbad and Rust SDK 2f039993d1d6ed8da35b38c31f54a7cbb7338c70.

Consumer

• Kit: feat(acp): add v2 mid-turn steering speakeasy-api/kit#24