test(observability-proxy): prepare seed stubs in both unit runners

The POSIX runner does not invoke Bats setup hooks. Initialize the
capture stub from the test body so the compatibility lane stays offline.

Fixes: 5a2582e21c ("test(observability-proxy): preserve JSON seed request bodies")
Assisted-by: LLM
Signed-off-by: Aleksei Sviridkin <f@lex.la>

hack/observability-proxy-seed.bats

@@ -4,13 +4,14 @@ load test_helper
 
 REPO_ROOT="$(cd "$(dirname "${BATS_TEST_FILENAME:-$0}")/.." && pwd)"
 
-setup() {
+prepare_seed_capture() {
     strict_setup
-    mkdir -p "$BATS_TEST_TMPDIR/bin"
-    export SEED_CAPTURE="$BATS_TEST_TMPDIR/seed"
-    export TMPDIR="$BATS_TEST_TMPDIR"
-    export PATH="$BATS_TEST_TMPDIR/bin:$PATH"
-    cat > "$BATS_TEST_TMPDIR/bin/curl" <<'EOF'
+    seed_tmp=$(mktemp -d)
+    seed_old_path=$PATH
+    mkdir -p "$seed_tmp/bin"
+    export SEED_CAPTURE="$seed_tmp/seed"
+    export PATH="$seed_tmp/bin:$PATH"
+    cat > "$seed_tmp/bin/curl" <<'EOF'
 #!/usr/bin/env bash
 set -eu
 printf '%s\n' "$@" > "$SEED_CAPTURE.args"
@@ -20,11 +21,12 @@ if ! grep --fixed-strings --line-regexp --quiet 'Content-Type: application/json'
     exit 22
 fi
 EOF
-    chmod +x "$BATS_TEST_TMPDIR/bin/curl"
+    chmod +x "$seed_tmp/bin/curl"
 }
 
 assert_seed_request() {
     local account=$1
+    prepare_seed_capture
     # shellcheck source=e2e-chainsaw/observability-proxy/lib.sh
     . "$REPO_ROOT/hack/e2e-chainsaw/observability-proxy/lib.sh"
     if [ "$account" = 0 ]; then
@@ -39,7 +41,9 @@ assert_seed_request() {
     grep --fixed-strings --line-regexp --quiet '@-' "$SEED_CAPTURE.args"
     printf '%s\n' '{"kubernetes_namespace_name":"tenant-test-logsa","log":"e2e-marker-a"}' > "$SEED_CAPTURE.expected"
     cmp "$SEED_CAPTURE.expected" "$SEED_CAPTURE.body"
-    rm "$body"
+    rm "$body" "$SEED_CAPTURE.args" "$SEED_CAPTURE.body" "$SEED_CAPTURE.expected" "$seed_tmp/bin/curl"
+    rmdir "$seed_tmp/bin" "$seed_tmp"
+    export PATH="$seed_old_path"
 }
 
 @test "observability seeds send JSON without consuming the body as form data" {