Address jrgemignani review: keep -Werror, restore %expect budget

Reverts the broad `-Werror` drop and the no-%expect approach from the
prior round on jrgemignani's request.  The earlier framing — that
conflict counts drift across Bison versions, so %expect is fragile —
overcorrected: it removed the only build-time alarm bell for unintended
new conflicts.

Makefile: keep -Werror so any unexpected Bison warning (unused rules,
undeclared types, etc.) still fails the build; downgrade only the two
conflict categories to plain warnings via -Wno-error=conflicts-sr
-Wno-error=conflicts-rr.  pgxs auto-adds -Wno-deprecated, so existing
%name-prefix= / %pure-parser directives remain non-erroring.

cypher_gram.y: add `%expect 7` and `%expect-rr 3` matching the
Bison 3.8.2 totals.  Bison treats %expect as exact-match, not as a
ceiling — any deviation fails the build and forces an audit of the new
conflicts.  Comment updated to reflect that future Bison versions
reporting different counts should bump the numbers explicitly with a
version note in the commit message, rather than removing the directive.

No grammar or runtime change.  Cassert installcheck 34/34 AGE tests
green.

Author: gregfelice

Makefile

@@ -229,12 +229,14 @@ src/include/parser/cypher_gram_def.h: src/backend/parser/cypher_gram.c
 # GLR handles these correctly at runtime by forking at the conflict
 # point; %dprec annotations resolve cases where both forks succeed.
 #
-# We suppress the conflict warnings rather than hard-coding a conflict
-# budget with %expect / %expect-rr, because the exact counts vary across
-# Bison versions and would otherwise make the build fragile across
-# distros and future Bison releases.
+# We keep -Werror so any unexpected Bison warning (unused rules, undeclared
+# types, etc.) still fails the build; we downgrade only the two conflict
+# categories to plain warnings via -Wno-error=.  The exact conflict totals
+# are pinned by %expect / %expect-rr in cypher_gram.y, which Bison treats
+# as exact-match: any deviation fails the build and forces an audit of
+# the new conflicts.
 #
-src/backend/parser/cypher_gram.c: BISONFLAGS += --defines=src/include/parser/cypher_gram_def.h -Wno-conflicts-sr -Wno-conflicts-rr
+src/backend/parser/cypher_gram.c: BISONFLAGS += --defines=src/include/parser/cypher_gram_def.h -Werror -Wno-error=conflicts-sr -Wno-error=conflicts-rr
 
 src/backend/parser/cypher_parser.o: src/backend/parser/cypher_gram.c src/include/parser/cypher_gram_def.h
 src/backend/parser/cypher_parser.bc: src/backend/parser/cypher_gram.c src/include/parser/cypher_gram_def.h

src/backend/parser/cypher_gram.y

@@ -71,12 +71,21 @@
  * anonymous_path resolve cases where both forks succeed (bare (a)
  * prefers the expression interpretation).
  *
- * We intentionally do not use %expect / %expect-rr here because the
- * exact conflict counts can vary across Bison versions (and across
- * distros) as the generator's internals change.  Instead, the Makefile
- * passes -Wno-conflicts-sr,-Wno-conflicts-rr via BISONFLAGS so the
- * build stays clean without binding us to a specific Bison release.
+ * The %expect / %expect-rr counts below match the Bison-reported totals
+ * (7 SR / 3 RR on Bison 3.8.2).  Bison treats %expect as exact, not as
+ * a ceiling: any deviation up or down fails the build.  That is the
+ * alarm bell — if a grammar change moves either count, the build stops
+ * and the conflicts must be audited to confirm they remain the inherent
+ * '(' expr ')' vs anonymous_path ambiguities (resolved by %dprec at
+ * runtime) rather than an unintended new ambiguity.  The Makefile
+ * downgrades -Wconflicts-sr / -Wconflicts-rr from errors to warnings
+ * (-Wno-error=conflicts-{sr,rr}) so %expect, not the warning category,
+ * controls the build-fail threshold.  If a future Bison version reports
+ * different counts for the same grammar, update these numbers and note
+ * the version in the commit message.
  */
+%expect 7
+%expect-rr 3
 
 %lex-param {ag_scanner_t scanner}
 %parse-param {ag_scanner_t scanner}