Discover HTTP MCP servers from selected executors (#39941)

## What changed

- Read `mcp_servers` configuration and requirements from each selected remote
  executor and add eligible HTTP servers to the thread's MCP runtime.
- Bind discovered servers to the thread's concrete executor snapshot and apply
  environment MCP policy and requirements. Discovery is best effort, and
  executor-local servers are not treated as required at startup.
- Ignore unsupported stdio servers and HTTP configurations that depend on
  environment-provided headers or header helpers.

## Testing

- Added an app-server integration test covering discovery, authenticated HTTP
  tool invocation, requirements enforcement, and exclusion of stdio servers.

GitOrigin-RevId: 6e1cdcebdbb1cc21a5a2285fbc5617d8d5997182

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

@@ -120,7 +120,7 @@ impl Session {
                 windows_sandbox_level,
             )
             .await;
-        let mcp_config = self
+        let mcp_projection = self
             .services
             .mcp_manager
             .runtime_config_for_step(
@@ -135,6 +135,9 @@ impl Session {
                 &ready_selected_capability_roots,
                 executor_capability_discovery.as_deref(),
             )
+            .await;
+        let mcp_config = self
+            .project_selected_environment_mcp_servers(config, &environments, mcp_projection)
             .await
             .config;
         let local_process_cwd = environments
@@ -144,6 +147,17 @@ impl Session {
         let runtime_context = McpRuntimeContext::new(
             self.services.turn_environments.environment_manager(),
             local_process_cwd,
+        )
+        .with_selected_environments(
+            environments
+                .turn_environments()
+                .map(|environment| {
+                    (
+                        environment.selection.environment_id.clone(),
+                        Arc::clone(&environment.environment),
+                    )
+                })
+                .collect(),
         );
         (mcp_config, runtime_context)
     }
@@ -265,6 +279,13 @@ impl Session {
                 executor_capability_discovery.as_deref(),
             )
             .await;
+        let mcp_projection = self
+            .project_selected_environment_mcp_servers(
+                &desired.config,
+                &desired.environments,
+                mcp_projection,
+            )
+            .await;
         let selected_plugins = mcp_projection.selected_plugins.clone();
         let input = self.build_mcp_runtime_input(
             &desired,