fix(integration): repair TestExpandedPostingsCacheFuzz latent bugs (#7549)

TestExpandedPostingsCacheFuzz had two independent defects that together
neutered the test:

1. The query-generation loop at integration/query_fuzz_test.go:550-557
   was inverted: `break` fired the moment isValidQuery returned true,
   exiting the outer `for i` loop, and only *invalid* expressions ever
   reached the append. The queries/matchers slices ended up holding at
   most testRun invalid exprs (or empty if the first draw was valid).
   Every downstream c1.Query / c1.QueryRange call then short-circuited
   with the same error on both clients, so cmp.Equal(err1, err2)
   trivially held and nothing was actually compared.

2. The Series diff branch at integration/query_fuzz_test.go:657 read
   `cmp.Equal(tc.sres1, tc.sres1, ...)` — self-comparison — so the
   Series API divergence check was dead code.

Net effect: the test had been a no-op for divergences. Issue #7545
captures the empirical census.

Fix: wrap the WalkRangeQuery call in a nested `for { ... }` that
re-draws until isValidQuery is satisfied, mirroring the sibling fuzz
tests in this file, and change `tc.sres1, tc.sres1` to
`tc.sres1, tc.sres2`. No production code touched; enabledAggrs is
already wired in.

Two follow-ups worth flagging honestly: wall-clock will grow
substantially (the previously-skipped ~9k HTTP round-trips now
actually run), and any genuine v1.18.0-vs-HEAD divergence in the
expanded-postings-cache code path will now surface as a real failure
for the first time, to be triaged as a real bug rather than a
regression from this PR.

Fixes #7545

Signed-off-by: Sandy Chen <Yuxuan.Chen@morganstanley.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: sandy2008

integration/query_fuzz_test.go

@@ -550,9 +550,12 @@ func TestExpandedPostingsCacheFuzz(t *testing.T) {
 	queries := make([]string, 0, testRun)
 	matchers := make([]string, 0, testRun)
 	for i := 0; i < testRun; i++ {
-		expr := ps.WalkRangeQuery()
-		if isValidQuery(expr, true) {
-			break
+		var expr parser.Expr
+		for {
+			expr = ps.WalkRangeQuery()
+			if isValidQuery(expr, true) {
+				break
+			}
 		}
 		queries = append(queries, expr.Pretty(0))
 		matchers = append(matchers, storepb.PromMatchersToString(
@@ -651,7 +654,7 @@ func TestExpandedPostingsCacheFuzz(t *testing.T) {
 			} else if !cmp.Equal(tc.res1, tc.res2, comparer) {
 				t.Logf("case %d results mismatch.\n%s: %s\nres1: %s\nres2: %s\n", i, tc.qt, tc.query, tc.res1.String(), tc.res2.String())
 				failures++
-			} else if !cmp.Equal(tc.sres1, tc.sres1, labelSetsComparer) {
+			} else if !cmp.Equal(tc.sres1, tc.sres2, labelSetsComparer) {
 				t.Logf("case %d results mismatch.\n%s: %s\nsres1: %s\nsres2: %s\n", i, tc.qt, tc.query, tc.sres1, tc.sres2)
 				failures++
 			}