Fix MATCH on brand-new label after CREATE returning 0 rows (#2341)

* Fix MATCH on brand-new label after CREATE returning 0 rows (issue #2193)

When CREATE introduces a new label and a subsequent MATCH references it
(e.g., CREATE (:Person) WITH ... MATCH (p:Person)), the query returns
0 rows on first execution but works on the second.

Root cause: match_check_valid_label() in transform_cypher_match() runs
before transform_prev_cypher_clause() processes the predecessor chain.
Since CREATE has not yet executed its transform (which creates the label
table as a side effect), the label is not in the cache and the check
generates a One-Time Filter: false plan that returns no rows.

Fix: Skip the early label validity check when the predecessor clause
chain contains a data-modifying operation (CREATE, SET, DELETE, MERGE).
After transform_prev_cypher_clause() completes and any new labels exist
in the cache, run a deferred label check. If the labels are still
invalid at that point, generate an empty result via makeBoolConst(false).

This preserves the existing behavior for MATCH without DML predecessors
(e.g., MATCH-MATCH chains still get the early check and proper error
messages for invalid labels).

Depends on: PR #2340 (clause_chain_has_dml helper)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Address review feedback: fix variable registration for deferred label check

When the deferred label validity check (DML predecessor + non-existent
label) found an invalid label, the code skipped transform_match_pattern()
entirely, which meant MATCH-introduced variables were never registered
in the namespace. This would cause errors if a later clause referenced
those variables (e.g., RETURN p).

Fix: mirror the early-check strategy by injecting a paradoxical WHERE
(true = false) and always calling transform_match_pattern(). Variables
get registered normally; zero rows are returned via the impossible qual.

Also add ORDER BY to multi-row regression tests for deterministic output,
and add a test case for DML predecessor + non-existent label + returning
a MATCH-introduced variable.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Address Copilot review: DRY false-where helper, cache has_dml, ORDER BY in tests

- Factor duplicated WHERE true=false construction into
  make_false_where_clause() helper (used in both early and deferred
  label validation paths)
- Compute clause_chain_has_dml() once and reuse, avoiding repeated
  clause chain traversal
- Add ORDER BY to the single-CREATE City regression test for
  deterministic result ordering

* Address Copilot review: volatile false predicate, DML side-effect test

1. Prevent plan elimination of DML predecessor: replace constant
   (true = false) with volatile (random() IS NULL) in the deferred
   label check path. PG's planner can constant-fold the former into
   a One-Time Filter: false, skipping the DML scan entirely.

2. Unify make_false_where_clause(bool volatile_needed): merge the
   constant and volatile variants into a single parameterized
   function. Call sites are now self-documenting:
   - make_false_where_clause(false) for non-DML path
   - make_false_where_clause(true) for DML predecessor path

3. Document why add_volatile_wrapper() cannot be reused here (it
   operates post-transform at the Expr level and returns agtype,
   while the WHERE clause is built at the parse-tree level).

4. Add regression test verifying CREATE side effects persist when
   MATCH references a non-existent label after a DML predecessor.

All regression tests pass (cypher_match: ok).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Replace non-ASCII em dashes with -- in C comments

ASCII-only codebase convention; avoids encoding/tooling issues.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>