fix: tighten uv version parser and platform-gate SYSTEMROOT in test client (#411)

## Summary

Addresses two unresolved Copilot review comments from merged PR #410.

## Changes

- **uv version parser:** Major and minor components are now explicitly
validated as purely numeric. Pre-release suffix (e.g., `0a4`, `0rc1`) is
only allowed on the patch component (3rd+). Previously `3.12abc` passed
validation because the minor was the "last" component and only needed to
start with a digit. Added test case for this scenario.

- **SYSTEMROOT in test client:** Platform-gated with `#[cfg(windows)]`
and only set when `std::env::var("SYSTEMROOT")` succeeds. Previously
used `unwrap_or_default()` which silently set it to empty string on
non-Windows or when missing.

Follow-up to #410

Author: karthiknadig

crates/pet-uv/src/lib.rs

@@ -350,21 +350,33 @@ fn parse_version_from_uv_dir_name(dir_name: &str) -> Option<String> {
         return None;
     }
     // Verify at minimum X.Y format (e.g., "3.12" or "3.12.3").
-    // Components before the last must be purely numeric.
-    // The last component may have a pre-release suffix (e.g., "0a4", "0rc1").
+    // Major and minor must always be purely numeric.
+    // Only the patch component (3rd+) may have a pre-release suffix (e.g., "0a4", "0rc1").
     let components: Vec<&str> = version.split('.').collect();
     if components.len() < 2 {
         return None;
     }
-    // All components except the last must be purely numeric.
-    let all_but_last = &components[..components.len() - 1];
-    if !all_but_last
-        .iter()
-        .all(|c| !c.is_empty() && c.chars().all(|ch| ch.is_ascii_digit()))
-    {
+
+    let is_numeric = |c: &str| !c.is_empty() && c.chars().all(|ch| ch.is_ascii_digit());
+
+    // Major and minor must be purely numeric.
+    if !is_numeric(components[0]) || !is_numeric(components[1]) {
+        return None;
+    }
+
+    // For X.Y versions (no patch), we're done.
+    if components.len() == 2 {
+        return Some(version.to_string());
+    }
+
+    // Any components between minor and the last must be purely numeric.
+    let middle = &components[2..components.len() - 1];
+    if !middle.iter().all(|c| is_numeric(c)) {
         return None;
     }
-    // The last component must start with a digit (allows pre-release suffix like "0a4").
+
+    // The last component (patch or beyond) must start with a digit
+    // (allows pre-release suffix like "0a4").
     let last = components.last()?;
     if last.is_empty() || !last.starts_with(|ch: char| ch.is_ascii_digit()) {
         return None;
@@ -1141,6 +1153,11 @@ exclude = ["packages/legacy"]"#;
             parse_version_from_uv_dir_name("cpython-3.12abc.1-linux"),
             None
         );
+        // 2-component version with non-numeric minor must be rejected.
+        assert_eq!(
+            parse_version_from_uv_dir_name("cpython-3.12abc-linux-x86_64-gnu"),
+            None
+        );
     }
 
     #[test]

crates/pet/tests/jsonrpc_client.rs

@@ -95,20 +95,23 @@ pub struct PetJsonRpcClient {
 
 impl PetJsonRpcClient {
     pub fn spawn() -> Result<Self, String> {
-        let mut process = Command::new(env!("CARGO_BIN_EXE_pet"))
-            .arg("server")
+        let mut cmd = Command::new(env!("CARGO_BIN_EXE_pet"));
+        cmd.arg("server")
             .stdin(Stdio::piped())
             .stdout(Stdio::piped())
             .stderr(Stdio::piped())
             // Clear all inherited env vars to prevent host-specific tool
             // configuration from leaking into the test environment, then
             // restore only the minimum required for the OS to function.
             .env_clear()
-            .env("PATH", "")
-            .env(
-                "SYSTEMROOT",
-                std::env::var("SYSTEMROOT").unwrap_or_default(),
-            )
+            .env("PATH", "");
+        // On Windows, SYSTEMROOT is required for basic OS functionality
+        // (crypto, networking, etc.). Only set it when present.
+        #[cfg(windows)]
+        if let Ok(val) = std::env::var("SYSTEMROOT") {
+            cmd.env("SYSTEMROOT", val);
+        }
+        let mut process = cmd
             .spawn()
             .map_err(|e| format!("Failed to spawn pet server: {e}"))?;