Pick a test interpreter that can import pytest, not just one named python

With Windows finally installing its dependencies, the CI build job reached
the test step for the first time and died on
"msys64/mingw64/bin/python.exe: No module named pytest".

The MSYS2 toolchain is prepended to PATH so native wheels can build, and
MSYS2 ships its own python.exe. run_ci_tests.sh took the first `python` on
PATH, which was therefore MSYS2's. The determinism gate immediately before it
had passed on the same runner because it invokes the `pytest` executable,
which MSYS2 does not provide — so PATH resolution quietly sent the two
scripts to different interpreters.

Selection is now by capability rather than by name: try each candidate and
keep the first that can actually import pytest, failing loudly with the
resolved paths if none can. Unchanged on Linux and macOS, where the first
`python` is the right one.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01Vtx94nXDmNgXEeThUZP4QN

scripts/run_ci_tests.sh

@@ -7,12 +7,28 @@ if [[ "${TUI2_SMOKE_GATE:-0}" == "1" ]]; then
     ./scripts/tui2_check_smoke.sh
 fi
 
+# Pick an interpreter that can actually import pytest, not merely the first
+# one named "python" on PATH. On Windows the MSYS2 toolchain is prepended to
+# PATH so native wheels can build, and MSYS2 ships its own python.exe -- so a
+# bare `python` resolved to that one and the whole suite died with "No module
+# named pytest", after the determinism gate had already passed using the
+# `pytest` executable, which MSYS2 does not provide.
 if [[ -x ".venv/bin/python" ]]; then
     py_bin=".venv/bin/python"
-elif command -v python >/dev/null 2>&1; then
-    py_bin="python"
 else
-    py_bin="python3"
+    py_bin=""
+    for candidate in python python3; do
+        if command -v "$candidate" >/dev/null 2>&1 \
+            && "$candidate" -c "import pytest" >/dev/null 2>&1; then
+            py_bin="$candidate"
+            break
+        fi
+    done
+    if [[ -z "$py_bin" ]]; then
+        echo "no interpreter on PATH can import pytest" >&2
+        command -v python python3 >&2 || true
+        exit 1
+    fi
 fi
 
 if [[ "${DETERMINISM_GATE:-1}" == "1" ]]; then