[elicitation] Advertise new url elicitation capability when auth_elicitation is enabled. (#22188)

## Why

We've added support for auth elicitation behind the auth_elicitation
flag, but servers need to explicitly check the capability before it
decides to send elicitations in order to be backward compatible. This PR
adds the capability advertising conditioned on the flag.

## What changed

- Build `client_elicitation_capability` from the `AuthElicitation`
feature state.
- Thread that capability through MCP config, session startup, and
`McpConnectionManager` so RMCP initialization advertises the correct
elicitation support.
- Advertise both `form` and `url` elicitation when the feature is
enabled, and preserve the empty default capability when it is disabled.
- Add coverage for the feature-derived config shape and the advertised
initialization payload.

## Testing

- `cargo test -p codex-mcp`
- `cargo test -p codex-core
to_mcp_config_preserves_auth_elicitation_feature_from_config`
- `cargo test -p codex-core` *(currently fails outside this change in
`tools::handlers::multi_agents::tests::tool_handlers_cascade_close_and_resume_and_keep_explicitly_closed_subtrees_closed`
with a stack overflow after unrelated tests have started running)*

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

@@ -10,7 +10,6 @@ use crate::elicitation::elicitation_is_rejected_by_policy;
 use crate::rmcp_client::AsyncManagedClient;
 use crate::rmcp_client::ManagedClient;
 use crate::rmcp_client::StartupOutcomeError;
-use crate::rmcp_client::elicitation_capability_for_server;
 use crate::tools::ToolFilter;
 use crate::tools::ToolInfo;
 use crate::tools::filter_tools;
@@ -844,15 +843,27 @@ async fn list_all_tools_uses_startup_snapshot_when_client_startup_fails() {
 }
 
 #[test]
-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_eq!(capability, Some(ElicitationCapability::default()));
-        assert_eq!(
-            serde_json::to_value(capability).expect("serialize elicitation capability"),
-            serde_json::json!({})
-        );
-    }
+fn elicitation_capability_uses_2025_06_18_shape_for_form_only_support() {
+    let capability = Some(ElicitationCapability::default());
+    assert_eq!(
+        serde_json::to_value(capability).expect("serialize elicitation capability"),
+        serde_json::json!({})
+    );
+}
+
+#[test]
+fn elicitation_capability_advertises_url_support_when_enabled() {
+    let capability = Some(ElicitationCapability {
+        form: Some(rmcp::model::FormElicitationCapability::default()),
+        url: Some(rmcp::model::UrlElicitationCapability::default()),
+    });
+    assert_eq!(
+        serde_json::to_value(capability).expect("serialize elicitation capability"),
+        serde_json::json!({
+            "form": {},
+            "url": {},
+        })
+    );
 }
 
 #[test]