[MCP] Allow specifying cwd and additional env vars (#5246)

This makes stdio mcp servers more flexible by allowing users to specify
the cwd to run the server command from and adding additional environment
variables to be passed through to the server.

Example config using the test server in this repo:
```toml
[mcp_servers.test_stdio]
cwd = "/Users/<user>/code/codex/codex-rs"
command = "cargo"
args = ["run", "--bin", "test_stdio_server"]
env_vars = ["MCP_TEST_VALUE"]
```

@bolinfest I know you hate these env var tests but let's roll with this
for now. I may take a stab at the env guard + serial macro at some
point.

codex-rs/rmcp-client/src/utils.rs

@@ -74,9 +74,12 @@ where
 
 pub(crate) fn create_env_for_mcp_server(
     extra_env: Option<HashMap<String, String>>,
+    env_vars: &[String],
 ) -> HashMap<String, String> {
     DEFAULT_ENV_VARS
         .iter()
+        .copied()
+        .chain(env_vars.iter().map(String::as_str))
         .filter_map(|var| env::var(var).ok().map(|value| (var.to_string(), value)))
         .chain(extra_env.unwrap_or_default())
         .collect()
@@ -185,13 +188,59 @@ mod tests {
     use rmcp::model::CallToolResult as RmcpCallToolResult;
     use serde_json::json;
 
+    use serial_test::serial;
+    use std::ffi::OsString;
+
+    struct EnvVarGuard {
+        key: String,
+        original: Option<OsString>,
+    }
+
+    impl EnvVarGuard {
+        fn set(key: &str, value: &str) -> Self {
+            let original = std::env::var_os(key);
+            unsafe {
+                std::env::set_var(key, value);
+            }
+            Self {
+                key: key.to_string(),
+                original,
+            }
+        }
+    }
+
+    impl Drop for EnvVarGuard {
+        fn drop(&mut self) {
+            if let Some(value) = &self.original {
+                unsafe {
+                    std::env::set_var(&self.key, value);
+                }
+            } else {
+                unsafe {
+                    std::env::remove_var(&self.key);
+                }
+            }
+        }
+    }
+
     #[tokio::test]
     async fn create_env_honors_overrides() {
         let value = "custom".to_string();
-        let env = create_env_for_mcp_server(Some(HashMap::from([("TZ".into(), value.clone())])));
+        let env =
+            create_env_for_mcp_server(Some(HashMap::from([("TZ".into(), value.clone())])), &[]);
         assert_eq!(env.get("TZ"), Some(&value));
     }
 
+    #[test]
+    #[serial(extra_rmcp_env)]
+    fn create_env_includes_additional_whitelisted_variables() {
+        let custom_var = "EXTRA_RMCP_ENV";
+        let value = "from-env";
+        let _guard = EnvVarGuard::set(custom_var, value);
+        let env = create_env_for_mcp_server(None, &[custom_var.to_string()]);
+        assert_eq!(env.get(custom_var), Some(&value.to_string()));
+    }
+
     #[test]
     fn convert_call_tool_result_defaults_missing_content() -> Result<()> {
         let structured_content = json!({ "key": "value" });