fix: ensure cwd for conversation and sandbox are separate concerns (#3874)

Previous to this PR, both of these functions take a single `cwd`:


71038381aa/codex-rs/core/src/seatbelt.rs (L19-L25)


71038381aa/codex-rs/core/src/landlock.rs (L16-L23)

whereas `cwd` and `sandbox_cwd` should be set independently (fixed in
this PR).

Added `sandbox_distinguishes_command_and_policy_cwds()` to
`codex-rs/exec/tests/suite/sandbox.rs` to verify this.

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

@@ -589,12 +589,14 @@ impl CodexMessageProcessor {
         let codex_linux_sandbox_exe = self.config.codex_linux_sandbox_exe.clone();
         let outgoing = self.outgoing.clone();
         let req_id = request_id;
+        let sandbox_cwd = self.config.cwd.clone();
 
         tokio::spawn(async move {
             match codex_core::exec::process_exec_tool_call(
                 exec_params,
                 sandbox_type,
                 &effective_policy,
+                sandbox_cwd.as_path(),
                 &codex_linux_sandbox_exe,
                 None,
             )