Honor null thread instructions (#16964)

- Treat explicit null thread instructions as a blank-slate override
while preserving omitted-field fallback behavior.
- Preserve null through rollout resume/fork and keep explicit empty
strings distinct.
- Add app-server v2 start/fork coverage for the tri-state instruction
params.

codex-rs/mcp-server/src/codex_tool_config.rs

@@ -179,8 +179,8 @@ impl CodexToolCallParam {
             codex_self_exe: arg0_paths.codex_self_exe.clone(),
             codex_linux_sandbox_exe: arg0_paths.codex_linux_sandbox_exe.clone(),
             main_execve_wrapper_exe: arg0_paths.main_execve_wrapper_exe.clone(),
-            base_instructions,
-            developer_instructions,
+            base_instructions: base_instructions.map(Some),
+            developer_instructions: developer_instructions.map(Some),
             compact_prompt,
             ..Default::default()
         };