Add SET n:Label and REMOVE n:Label (#2281)

NOTE: This PR was built with AI tools and verified by a human.

Implements Cypher SET and REMOVE operations for vertex labels in the unified
vertex table architecture. This allows dynamic label management on vertices.

SET n:Label

* Only works on vertices with no label.

* Auto-creates the label if it doesn't exist.

* Errors with hint if vertex already has a label:
"Multiple labels are not supported. Use REMOVE to clear the label first."

REMOVE n:Label

* Removes a vertex's specified label.

* Properties are preserved

* No-op if vertex already has no label

Added regression tests.

modified:   regress/expected/cypher_remove.out
modified:   regress/expected/unified_vertex_table.out
modified:   regress/sql/unified_vertex_table.sql
modified:   src/backend/executor/cypher_set.c
modified:   src/backend/nodes/cypher_copyfuncs.c
modified:   src/backend/nodes/cypher_outfuncs.c
modified:   src/backend/nodes/cypher_readfuncs.c
modified:   src/backend/parser/cypher_clause.c
modified:   src/backend/parser/cypher_gram.y
modified:   src/include/nodes/cypher_nodes.h

Author: jrgemignani

regress/expected/cypher_remove.out

@@ -463,7 +463,7 @@ ERROR:  REMOVE cannot be the first clause in a Cypher query
 LINE 1: SELECT * FROM cypher('cypher_remove', $$REMOVE n.i$$) AS (a ...
                                                 ^
 SELECT * FROM cypher('cypher_remove', $$MATCH (n) REMOVE n.i = NULL$$) AS (a agtype);
-ERROR:  REMOVE clause must be in the format: REMOVE variable.property_name
+ERROR:  REMOVE clause must be in the format: REMOVE variable.property_name or REMOVE variable:Label
 LINE 1: SELECT * FROM cypher('cypher_remove', $$MATCH (n) REMOVE n.i...
                                                 ^
 SELECT * FROM cypher('cypher_remove', $$MATCH (n) REMOVE wrong_var.i$$) AS (a agtype);