chore: introduce SandboxPolicy::WorkspaceWrite::use_exact_writable_roots

Without this change, it is challenging to create integration tests to
verify that the folders not included in `writable_roots` in
`SandboxPolicy::WorkspaceWrite` are read-only because, by default,
`get_writable_roots_with_cwd()` includes `TMPDIR`, which is where most integrationt
tests do their work.

This introduces a `use_exact_writable_roots` option to disable the default
includes returned by `get_writable_roots_with_cwd()`.

codex-rs/common/src/sandbox_summary.rs

@@ -7,6 +7,7 @@ pub fn summarize_sandbox_policy(sandbox_policy: &SandboxPolicy) -> String {
         SandboxPolicy::WorkspaceWrite {
             writable_roots,
             network_access,
+            use_exact_writable_roots,
         } => {
             let mut summary = "workspace-write".to_string();
             if !writable_roots.is_empty() {
@@ -19,6 +20,9 @@ pub fn summarize_sandbox_policy(sandbox_policy: &SandboxPolicy) -> String {
                         .join(", ")
                 ));
             }
+            if *use_exact_writable_roots {
+                summary.push_str(" (exact writable roots)");
+            }
             if *network_access {
                 summary.push_str(" (network access enabled)");
             }

codex-rs/core/src/config.rs

@@ -350,6 +350,7 @@ impl ConfigToml {
                 Some(s) => SandboxPolicy::WorkspaceWrite {
                     writable_roots: s.writable_roots.clone(),
                     network_access: s.network_access,
+                    use_exact_writable_roots: false,
                 },
                 None => SandboxPolicy::new_workspace_write_policy(),
             },
@@ -720,6 +721,7 @@ writable_roots = [
             SandboxPolicy::WorkspaceWrite {
                 writable_roots: vec![PathBuf::from("/tmp")],
                 network_access: false,
+                use_exact_writable_roots: false,
             },
             sandbox_workspace_write_cfg.derive_sandbox_policy(sandbox_mode_override)
         );

codex-rs/core/src/protocol.rs

@@ -175,6 +175,10 @@ pub enum SandboxPolicy {
         /// default.
         #[serde(default)]
         network_access: bool,
+
+        /// When set to `true`, will not include defaults like TMPDIR.
+        /// (Mainly used for testing.)
+        use_exact_writable_roots: bool,
     },
 }
 
@@ -199,6 +203,7 @@ impl SandboxPolicy {
         SandboxPolicy::WorkspaceWrite {
             writable_roots: vec![],
             network_access: false,
+            use_exact_writable_roots: false,
         }
     }
 
@@ -230,7 +235,15 @@ impl SandboxPolicy {
         match self {
             SandboxPolicy::DangerFullAccess => Vec::new(),
             SandboxPolicy::ReadOnly => Vec::new(),
-            SandboxPolicy::WorkspaceWrite { writable_roots, .. } => {
+            SandboxPolicy::WorkspaceWrite {
+                writable_roots,
+                use_exact_writable_roots,
+                ..
+            } => {
+                if *use_exact_writable_roots {
+                    return writable_roots.clone();
+                }
+
                 let mut roots = writable_roots.clone();
                 roots.push(cwd.to_path_buf());