Forward unstable_mcp_over_acp feature through SDK crate

Instead of unconditionally enabling unstable_mcp_over_acp on the schema
dependency, declare it as a feature on agent-client-protocol that
forwards to agent-client-protocol-schema/unstable_mcp_over_acp.

Conductor and polyfill crates opt in explicitly.

Author: nikomatsakis

Cargo.toml

@@ -34,7 +34,7 @@ tokio = { version = "1.48", features = ["full"] }
 tokio-util = { version = "0.7", features = ["compat"] }
 
 # Protocol
-agent-client-protocol-schema = { path = "../agent-client-protocol", features = ["tracing", "unstable_mcp_over_acp"] }
+agent-client-protocol-schema = { path = "../agent-client-protocol", features = ["tracing"] }
 
 # Serialization
 serde = { version = "1.0", features = ["derive", "rc"] }

src/agent-client-protocol-conductor/Cargo.toml

@@ -18,7 +18,7 @@ path = "src/main.rs"
 test-support = []
 
 [dependencies]
-agent-client-protocol.workspace = true
+agent-client-protocol = { workspace = true, features = ["unstable_mcp_over_acp"] }
 agent-client-protocol-schema.workspace = true
 agent-client-protocol-trace-viewer.workspace = true
 anyhow.workspace = true

src/agent-client-protocol-polyfill/Cargo.toml

@@ -11,7 +11,7 @@ keywords = ["acp", "agent", "mcp", "polyfill"]
 categories = ["development-tools"]
 
 [dependencies]
-agent-client-protocol.workspace = true
+agent-client-protocol = { workspace = true, features = ["unstable_mcp_over_acp"] }
 agent-client-protocol-schema.workspace = true
 anyhow.workspace = true
 async-stream.workspace = true

src/agent-client-protocol/Cargo.toml

@@ -19,6 +19,7 @@ unstable = [
     "unstable_auth_methods",
     "unstable_boolean_config",
     "unstable_logout",
+    "unstable_mcp_over_acp",
     "unstable_message_id",
     "unstable_session_additional_directories",
     "unstable_session_close",
@@ -30,6 +31,7 @@ unstable = [
 unstable_auth_methods = ["agent-client-protocol-schema/unstable_auth_methods"]
 unstable_boolean_config = ["agent-client-protocol-schema/unstable_boolean_config"]
 unstable_logout = ["agent-client-protocol-schema/unstable_logout"]
+unstable_mcp_over_acp = ["agent-client-protocol-schema/unstable_mcp_over_acp"]
 unstable_message_id = ["agent-client-protocol-schema/unstable_message_id"]
 unstable_session_additional_directories = ["agent-client-protocol-schema/unstable_session_additional_directories"]
 unstable_session_close = ["agent-client-protocol-schema/unstable_session_close"]