fix(mosaic): route the Invariant R pi version probe through the bounded-retry runner

The version probe was a bare subprocess.run with a 10s timeout while the
registry probe already had run_pi_registry_command's 45s timeout and three
attempts for concurrent-Pi stalls. Under CI queue load the bare probe is the
step that times out: pipelines 2808-2810 and 2819-2820 all failed at
test_pi_carve_out_resolves_to_real_unshadowed_builtins with
'pi --version timed out after 10 seconds' on unrelated (including docs-only)
heads, while 2814/2816 passed between them. Reusing the existing runner gives
the version probe the same bounded retries and the same probe/infra failure
labeling; behavior on a version mismatch or nonzero exit is unchanged.

packages/mosaic/src/lease-broker/invariant_r_unittest.py

@@ -75,7 +75,7 @@ def run_pi_registry_command(
     runner=subprocess.run,
     sleeper=time.sleep,
 ) -> subprocess.CompletedProcess[str]:
-    """Run the registry probe with bounded retries for concurrent-Pi stalls."""
+    """Run a Pi probe command with bounded retries for concurrent-Pi stalls."""
 
     for attempt in range(1, PI_PROBE_ATTEMPTS + 1):
         try:
@@ -106,13 +106,7 @@ def probe_pi_registry() -> list[dict[str, object]]:
     if pi is None:
         raise AssertionError("installed Pi runtime is required for Invariant R")
 
-    version = subprocess.run(
+    version = run_pi_registry_command([pi, "--version"], dict(os.environ))
-        [pi, "--version"],
-        check=False,
-        capture_output=True,
-        text=True,
-        timeout=10,
-    )
     if version.returncode != 0:
         raise AssertionError(f"Pi version probe failed: {version.stderr.strip()}")
     if version.stdout.strip() != PI_VERSION: