Use snapshot session sources for MCP projection (#40368)

## What changed

Pass the session source associated with each MCP configuration snapshot into
selected-environment server projection. This keeps Guardian reviewer isolation
decisions aligned with the snapshot being projected instead of rereading the
session's mutable configuration.

GitOrigin-RevId: 61fd891fec13f62cfea9886d13e2f94f6f60922f

codex-rs/core/src/session/mcp.rs

@@ -137,7 +137,12 @@ impl Session {
             )
             .await;
         let mcp_config = self
-            .project_selected_environment_mcp_servers(config, &environments, mcp_projection)
+            .project_selected_environment_mcp_servers(
+                &session_source,
+                config,
+                &environments,
+                mcp_projection,
+            )
             .await
             .config;
         let local_process_cwd = environments
@@ -281,6 +286,7 @@ impl Session {
             .await;
         let mcp_projection = self
             .project_selected_environment_mcp_servers(
+                &desired.session_source,
                 &desired.config,
                 &desired.environments,
                 mcp_projection,