feat: use ProcessId in exec-server (#15866)

Use a full struct for the ProcessId to increase readability and make it
easier in the future to make it evolve if needed

codex-rs/exec-server/src/process_id.rs

@@ -0,0 +1,74 @@
+use std::borrow::Borrow;
+use std::fmt;
+use std::ops::Deref;
+
+use serde::Deserialize;
+use serde::Serialize;
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, Serialize, Deserialize)]
+#[serde(transparent)]
+pub struct ProcessId(String);
+
+impl ProcessId {
+    pub fn new(value: impl Into<String>) -> Self {
+        Self(value.into())
+    }
+
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+
+    pub fn into_inner(self) -> String {
+        self.0
+    }
+}
+
+impl Deref for ProcessId {
+    type Target = str;
+
+    fn deref(&self) -> &Self::Target {
+        self.as_str()
+    }
+}
+
+impl Borrow<str> for ProcessId {
+    fn borrow(&self) -> &str {
+        self.as_str()
+    }
+}
+
+impl AsRef<str> for ProcessId {
+    fn as_ref(&self) -> &str {
+        self.as_str()
+    }
+}
+
+impl fmt::Display for ProcessId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+impl From<String> for ProcessId {
+    fn from(value: String) -> Self {
+        Self(value)
+    }
+}
+
+impl From<&str> for ProcessId {
+    fn from(value: &str) -> Self {
+        Self(value.to_string())
+    }
+}
+
+impl From<&String> for ProcessId {
+    fn from(value: &String) -> Self {
+        Self(value.clone())
+    }
+}
+
+impl From<ProcessId> for String {
+    fn from(value: ProcessId) -> Self {
+        value.0
+    }
+}