fix(tt4): drop the dead pid helper so e2e_pty is warning-free off Linux

`rss_kib` is Linux-only (#[cfg(target_os = "linux")]) and was the sole
caller of `pid()`, so on macOS/Windows `pid` was dead code and emitted a
`method `pid` is never used` warning. The Linux CI clippy gate never saw
it — `pid` is used on Linux — so it only surfaced on a manual macOS run.
Read `child.process_id()` inline in `rss_kib` and delete the helper.

Verified: macOS `cargo test` is fully green (5 e2e_pty tests there; the
Linux-only RSS test is cfg'd out). clippy --all-targets + fmt clean.

tests/e2e_pty.rs

@@ -233,14 +233,13 @@ impl PtyApp {
         self.send(b"\r");
     }
 
-    fn pid(&self) -> Option<u32> {
-        self.child.process_id()
-    }
-
-    /// Resident set size of the child in KiB (Linux only).
+    /// Resident set size of the child in KiB (Linux only). `process_id`
+    /// is read inline here rather than via a helper so the method has no
+    /// non-Linux callers to go dead (the CI clippy gate runs on Linux,
+    /// where such dead code wouldn't surface).
     #[cfg(target_os = "linux")]
     fn rss_kib(&self) -> Option<u64> {
-        let pid = self.pid()?;
+        let pid = self.child.process_id()?;
         let status = std::fs::read_to_string(format!("/proc/{pid}/status")).ok()?;
         status.lines().find_map(|l| {
             let rest = l.strip_prefix("VmRSS:")?;