[codex] Apply patches through executor filesystem (#17048)

## Summary
- run apply_patch through the executor filesystem when a remote
environment is present instead of shelling out to the local process
- thread the executor FileSystem into apply_patch interception and keep
existing local behavior for non-remote turns
- make the apply_patch integration harness use the executor filesystem
for setup/assertions
- add remote-aware skips for turn-diff coverage that still reads the
test-runner filesystem

## Why
Remote apply_patch needed to mutate the remote workspace instead of the
local checkout. The tests also needed to seed and assert workspace state
through the same filesystem abstraction so local and remote runs
exercise the same behavior.

## Validation
- `just fmt`
- `git diff --check`
- `cargo check -p core_test_support --tests`
- `cargo test -p codex-core --test all
suite::shell_serialization::apply_patch_custom_tool_call -- --nocapture`
- `cargo test -p codex-core --test all
suite::apply_patch_cli::apply_patch_cli_updates_file_appends_trailing_newline
-- --nocapture`
- remote `cargo test -p codex-core --test all apply_patch_cli --
--nocapture` (229 passed)

codex-rs/core/tests/common/lib.rs

@@ -303,6 +303,10 @@ pub fn sandbox_network_env_var() -> &'static str {
 
 const REMOTE_ENV_ENV_VAR: &str = "CODEX_TEST_REMOTE_ENV";
 
+pub fn remote_env_env_var() -> &'static str {
+    REMOTE_ENV_ENV_VAR
+}
+
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct RemoteEnvConfig {
     pub container_name: String,
@@ -537,6 +541,30 @@ macro_rules! skip_if_no_network {
     }};
 }
 
+#[macro_export]
+macro_rules! skip_if_remote {
+    ($reason:expr $(,)?) => {{
+        if ::std::env::var_os($crate::remote_env_env_var()).is_some() {
+            eprintln!(
+                "Skipping test under {}: {}",
+                $crate::remote_env_env_var(),
+                $reason
+            );
+            return;
+        }
+    }};
+    ($return_value:expr, $reason:expr $(,)?) => {{
+        if ::std::env::var_os($crate::remote_env_env_var()).is_some() {
+            eprintln!(
+                "Skipping test under {}: {}",
+                $crate::remote_env_env_var(),
+                $reason
+            );
+            return $return_value;
+        }
+    }};
+}
+
 #[macro_export]
 macro_rules! codex_linux_sandbox_exe_or_skip {
     () => {{