Use the 2025-06-18 elicitation capability shape (#20562)

# Why

Codex currently negotiates MCP `2025-06-18`, where the client
elicitation capability is represented as an empty object. We were still
serializing `capabilities.elicitation.form`, which belongs to the later
capability shape and can cause strict `2025-06-18` servers to reject
`initialize` with an unrecognized-field error.

This keeps the handshake aligned with the protocol version Codex
actually negotiates and fixes the compatibility regression tracked in
#17492.

# What

- Serialize the client elicitation capability as `elicitation: {}` for
`2025-06-18`.
- Keep elicitation advertised for both Codex Apps and custom MCP
servers.
- Tighten regression coverage so the unit test asserts both the Rust
value and the serialized wire shape.
- Add an app-server integration test that round-trips a form elicitation
from a custom MCP server; the existing connector round-trip continues to
cover the connector path.

# Verification

- `cargo test -p codex-mcp`
- `cargo test -p codex-app-server mcp_server_elicitation_round_trip`
- `cargo test -p codex-app-server
mcp_server_tool_call_round_trips_elicitation`

# Next steps

- Decide whether `tool_call_mcp_elicitation=false` should also suppress
capability advertisement during `initialize`.
- Revisit `form` / `url` capability advertisement when Codex is ready to
negotiate MCP `2025-11-25`, which defines that newer shape.

codex-rs/codex-mcp/src/connection_manager_tests.rs

@@ -26,7 +26,6 @@ use pretty_assertions::assert_eq;
 use rmcp::model::CreateElicitationRequestParams;
 use rmcp::model::ElicitationAction;
 use rmcp::model::ElicitationCapability;
-use rmcp::model::FormElicitationCapability;
 use rmcp::model::JsonObject;
 use rmcp::model::Meta;
 use rmcp::model::NumberOrString;
@@ -801,18 +800,14 @@ async fn list_all_tools_uses_startup_snapshot_when_client_startup_fails() {
 }
 
 #[test]
-fn elicitation_capability_enabled_for_custom_servers() {
+fn elicitation_capability_uses_2025_06_18_shape_for_all_servers() {
     for server_name in [CODEX_APPS_MCP_SERVER_NAME, "custom_mcp"] {
         let capability = elicitation_capability_for_server(server_name);
-        assert!(matches!(
-            capability,
-            Some(ElicitationCapability {
-                form: Some(FormElicitationCapability {
-                    schema_validation: None
-                }),
-                url: None,
-            })
-        ));
+        assert_eq!(capability, Some(ElicitationCapability::default()));
+        assert_eq!(
+            serde_json::to_value(capability).expect("serialize elicitation capability"),
+            serde_json::json!({})
+        );
     }
 }