[codex] wire process-owned code mode host into core (#30142)

## Summary

- add the `code_mode_host` feature flag and select
`ProcessOwnedCodeModeSessionProvider` in `CodeModeService` when enabled
- initialize code-mode sessions lazily so a missing host reports a tool
error without failing thread startup
- resolve `codex-code-mode-host` beside the running Codex binary by
default while preserving `CODEX_CODE_MODE_HOST_PATH` as an override
- add unit and end-to-end coverage for host resolution and graceful
missing-host behavior

## Why

This wires the process-owned session client from #30112 into the core
service behind an opt-in rollout gate. Packaged Codex installations can
place the helper in the same `bin` directory as the main executable
without relying on `PATH`, while development and custom installations
can continue to override the helper path.

## Stack

- Depends on #30112
- Base branch: `cconger/process-owned-session-runtime-4-client`

## Validation

Build `codex` and `codex-code-mode-host`
`CODEX_CODE_MODE_HOST_PATH="$PWD/target/debug/codex-code-mode-host"
./target/debug/codex --enable code_mode_host`

codex-rs/features/src/lib.rs

@@ -92,6 +92,8 @@ pub enum Feature {
     // Experimental
     /// Enable JavaScript code mode backed by the in-process V8 runtime.
     CodeMode,
+    /// Run JavaScript code mode in the standalone host process.
+    CodeModeHost,
     /// Restrict model-visible tools to code mode entrypoints (`exec`, `wait`).
     CodeModeOnly,
     /// Use the single unified PTY-backed exec tool.
@@ -850,6 +852,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::CodeModeHost,
+        key: "code_mode_host",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::CodeModeOnly,
         key: "code_mode_only",