fix: prepare ExecPolicy in exec-server for execpolicy2 cutover (#6888)

This PR introduces an extra layer of abstraction to prepare us for the
migration to execpolicy2:

- introduces a new trait, `EscalationPolicy`, whose `determine_action()`
method is responsible for producing the `EscalateAction`
- the existing `ExecPolicy` typedef is changed to return an intermediate
`ExecPolicyOutcome` instead of `EscalateAction`
- the default implementation of `EscalationPolicy`,
`McpEscalationPolicy`, composes `ExecPolicy`
- the `ExecPolicyOutcome` includes `codex_execpolicy2::Decision`, which
has a `Prompt` variant
- when `McpEscalationPolicy` gets `Decision::Prompt` back from
`ExecPolicy`, it prompts the user via an MCP elicitation and maps the
result into an `ElicitationAction`
- now that the end user can reply to an elicitation with `Decline` or
`Cancel`, we introduce a new variant, `EscalateAction::Deny`, which the
client handles by returning exit code `1` without running anything

Note the way the elicitation is created is still not quite right, but I
will fix that once we have things running end-to-end for real in a
follow-up PR.

codex-rs/exec-server/src/posix/mcp.rs

@@ -18,9 +18,10 @@ use rmcp::tool_handler;
 use rmcp::tool_router;
 use rmcp::transport::stdio;
 
-use crate::posix::escalate_server;
 use crate::posix::escalate_server::EscalateServer;
-use crate::posix::escalate_server::ExecPolicy;
+use crate::posix::escalate_server::{self};
+use crate::posix::mcp_escalation_policy::ExecPolicy;
+use crate::posix::mcp_escalation_policy::McpEscalationPolicy;
 
 /// Path to our patched bash.
 const CODEX_BASH_PATH_ENV_VAR: &str = "CODEX_BASH_PATH";
@@ -81,10 +82,13 @@ impl ExecTool {
     #[tool]
     async fn shell(
         &self,
-        _context: RequestContext<RoleServer>,
+        context: RequestContext<RoleServer>,
         Parameters(params): Parameters<ExecParams>,
     ) -> Result<CallToolResult, McpError> {
-        let escalate_server = EscalateServer::new(self.bash_path.clone(), self.policy);
+        let escalate_server = EscalateServer::new(
+            self.bash_path.clone(),
+            McpEscalationPolicy::new(self.policy, context),
+        );
         let result = escalate_server
             .exec(
                 params.command,
@@ -99,27 +103,6 @@ impl ExecTool {
             ExecResult::from(result),
         )?]))
     }
-
-    #[allow(dead_code)]
-    async fn prompt(
-        &self,
-        command: String,
-        workdir: String,
-        context: RequestContext<RoleServer>,
-    ) -> Result<CreateElicitationResult, McpError> {
-        context
-            .peer
-            .create_elicitation(CreateElicitationRequestParam {
-                message: format!("Allow Codex to run `{command:?}` in `{workdir:?}`?"),
-                #[allow(clippy::expect_used)]
-                requested_schema: ElicitationSchema::builder()
-                    .property("dummy", PrimitiveSchema::String(StringSchema::new()))
-                    .build()
-                    .expect("failed to build elicitation schema"),
-            })
-            .await
-            .map_err(|e| McpError::internal_error(e.to_string(), None))
-    }
 }
 
 #[tool_handler]