Add a feature flag for code-mode host output truncation (#58159)

## What changed

Add the disabled-by-default `code_mode_host_output_truncation` feature flag. When enabled, Core requests server-side output truncation for its built-in code-mode session providers over both stdio and gRPC. Injected providers retain control of their session settings, and explicit session options pass through unchanged.

## Testing

Add coverage for both transports verifying that the host's output-limit notice survives a zero-token observation budget. Extend the shared-provider test to exercise an injected provider with the new flag enabled.

GitOrigin-RevId: edbb65e930bf544e3d2f8384767037a6fb82d451

codex-rs/features/src/lib.rs

@@ -150,6 +150,8 @@ pub enum Feature {
     CodeModeHost,
     /// Use gRPC over stdio for the process-owned code-mode host.
     CodeModeHostGrpc,
+    /// Truncate output in the process-owned code-mode host before sending it to Core.
+    CodeModeHostOutputTruncation,
     /// Establish the code-mode host connection during session startup.
     CodeModePrewarm,
     /// Terminate active code mode cells when their turn is interrupted.
@@ -1281,6 +1283,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::CodeModeHostOutputTruncation,
+        key: "code_mode_host_output_truncation",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::CodeModePrewarm,
         key: "code_mode_prewarm",