tests: fix SIGPIPE under pipefail; use -print -quit instead of | head -1

`set -o pipefail` at the top of tests.sh combined with `find ... | head -1`
is a silent-killer pattern. When `find` produces lots of matches (e.g.
when uv has installed dozens of third-party packages with test_*.py
files under .venv), `head -1` reads its single line and closes the pipe.
`find` writes again, gets SIGPIPE, exits 141. pipefail propagates 141.
`set -e` fires. tests.sh exits 141 before printing `-- Python (pytest ...) --`
or any other output. run-all.sh records `tests: failed` with no
indication of WHY.

Reproduced directly:
    bash -c 'set -euo pipefail; x=$(find <large-tree> -name "test_*.py" -o -name "*_test.py" | head -1); echo "got: $x"'
    exit=141              # "got:" never prints

Fix: replace `| head -1` with `-print -quit` so find stops itself
after the first match instead of getting cut off mid-write. As a free
bonus, also prune the well-known noise dirs (.venv, .git, etc.) so the
probe doesn't even consider third-party test files in the first place —
this keeps the probe fast on dense trees and avoids matching
@pytest.fixture-decorated files in installed packages.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

validate/tests.sh

@@ -11,8 +11,18 @@ errors=0
 
 echo "=== Test Suite ==="
 
-has_python=$(find "$TARGET" -name "test_*.py" -o -name "*_test.py" | head -1)
-has_go=$(find "$TARGET" -name "*_test.go" | head -1)
+# Probe for test files. Prune known noise dirs so we don't trip on
+# third-party test files installed under .venv, and use -print -quit so
+# the probe doesn't SIGPIPE under `set -o pipefail` on dense trees
+# (which exits the script silently with status 141 before any stage
+# output is printed).
+has_python=$(find "$TARGET" \
+    \( -path '*/__pycache__' -o -path '*/.venv' -o -path '*/venv' \
+       -o -path '*/.git' -o -path '*/node_modules' \) -prune \
+    -o \( -name 'test_*.py' -o -name '*_test.py' \) -print -quit)
+has_go=$(find "$TARGET" \
+    \( -path '*/.git' -o -path '*/vendor' -o -path '*/node_modules' \) -prune \
+    -o -name '*_test.go' -print -quit)
 
 if [ -z "$has_python" ] && [ -z "$has_go" ]; then
   printf "${YELLOW}!${NC} No test files found — skipping\n"