Only enable unstable_mcp_over_acp in test builds

The conductor and polyfill don't reference McpServer::Acp or
McpCapabilities.acp in their source — only snapshot tests see the
field in serialized output. Move the feature to dev-dependencies.

Author: nikomatsakis

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

@@ -18,7 +18,7 @@ path = "src/main.rs"
 test-support = []
 
 [dependencies]
-agent-client-protocol = { workspace = true, features = ["unstable_mcp_over_acp"] }
+agent-client-protocol = { workspace = true }
 agent-client-protocol-schema.workspace = true
 agent-client-protocol-trace-viewer.workspace = true
 anyhow.workspace = true
@@ -44,6 +44,7 @@ tracing-subscriber.workspace = true
 uuid.workspace = true
 
 [dev-dependencies]
+agent-client-protocol = { workspace = true, features = ["unstable_mcp_over_acp"] }
 agent-client-protocol-test.workspace = true
 yopo.workspace = true
 expect-test.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, features = ["unstable_mcp_over_acp"] }
+agent-client-protocol = { workspace = true }
 agent-client-protocol-schema.workspace = true
 anyhow.workspace = true
 async-stream.workspace = true