feat(unstable-v2): add session injection support

[danielkov]

Summary

Adds Rust SDK support for the unstable ACP v2 session-injection schema.

• forwards the unstable_session_inject feature to the schema crate
• maps inject, revoke, and replace requests into the v2 JSON-RPC dispatcher
• adds typed v2 session helpers for sending those requests
• adds schema serialization and client/server session tests

Dependencies

• Schema: feat(unstable-v2): add session injection schema agent-client-protocol#2043
• Response receipts: feat(jsonrpc): add tracked response receipts #338

The branch currently pins the schema fork at 6e7e044f9464c4fd652d90699a09e9edc8b3bbad and includes the response-receipt patch. It will be rebased onto the upstream versions as those dependencies land.

Related

• RFD: docs(rfd): mid-turn input via session/inject (queue and steer) agent-client-protocol#1261
• AgentKit implementation: feat(acp): add unstable v2 session injection danielkov/agentkit#13
• Kit integration: feat(acp): add v2 mid-turn steering speakeasy-api/kit#24