fix(snap-tests): assert_grep — use here-string to dodge pipefail SIGPIPE

`assert_grep` ran `printf '%s' "$out" | grep -q -F -- "$needle"` under
`set -o pipefail`. When grep matched early it closed its stdin, printf
got SIGPIPE on its next write (exit 141), and pipefail propagated the
broken-pipe failure to the pipeline — making `if` see non-zero and
falling into the FAIL branch even though grep itself succeeded.

Failure was timing-dependent: it only fired when `$out` was large enough
that printf hadn't flushed before grep exited. CI ubuntu-latest tipped
into the racy path on PR #7698 once `settings.json.template` grew by 11
lines (the new `enablePluginPadOptions` flag); the symptom was the
`Wrapper unit tests` step reporting `dbType rewritten to sqlite ✗` with
"got: /*…" output even though the seeded file did contain the needle.

Replace the pipe with a here-string so grep gets its input in one shot
with no pipe between processes — no SIGPIPE possible. The fail-message
`head -3` is converted to a here-string for the same reason.

Repro on a runner whose pipe-buffer flush is slower than grep's first
match would have hit the same flake on any PR; the bug isn't about
this particular template change.

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

Author: JohnMcLear

snap/tests/lib.sh

@@ -57,14 +57,23 @@ assert_exit() {
 }
 
 # assert_grep cmd needle name — fail if cmd's combined output doesn't match
+#
+# Uses a here-string instead of `printf | grep -q` because `set -o pipefail`
+# (declared at the top of this file) propagates SIGPIPE failures: when grep
+# -q matches early it closes its stdin, printf gets SIGPIPE on its next
+# write, and pipefail makes the whole pipeline exit non-zero — even though
+# the grep itself succeeded. The failure mode is timing-dependent, only
+# tripping when the captured output is large enough that printf hasn't
+# flushed before grep matches and exits. A here-string feeds grep its input
+# in one shot with no pipe in between.
 assert_grep() {
   local needle="$1" name="$2"; shift 2
   local out
   out=$("$@" 2>&1 || true)
-  if printf '%s' "$out" | grep -q -F -- "$needle"; then
+  if grep -q -F -- "$needle" <<<"$out"; then
     pass "$name"
   else
-    fail "$name" "expected output to contain: $needle; got: $(printf '%s' "$out" | head -3)"
+    fail "$name" "expected output to contain: $needle; got: $(head -3 <<<"$out")"
   fi
 }