Add vertex and edge composite types with direct field access optimization

[MuhammadTahaNaveed]

• Introduce vertex and edge as pg composite types
  vertex: (id, label, properties)
  edge: (id, label, start_id, end_id, properties)
• Property access (a.name) now directly uses a.properties for agtype_access_operator instead of rebuilding via _agtype_build_vertex/edge
• Optimize accessor functions (id, properties, label, type, start_id, end_id) to use direct FieldSelect on composite types instead of agtype functions
• Add casts: vertex/edge to agtype, vertex/edge to json
• Fix label_name specific routine to use cache instead of ag_label scan
• Write/update clauses have executors strictly tied to agtype, due to which the variables after any write/update clause are carried forward as agtype.
• Allows users to completely skip agtype build functions and return vertex/edge for pure read queries.
• Change _label_name to return agtype since record comparisons are not allowed with cstring. Consequently, _agtype_build_vertex/edge now accept agtype as label.
• Fix MERGE clause type mismatch when accessing properties from previous MATCH clauses by wrapping columns with agtype_volatile_wrapper before namespace lookup.
• Update expression index in pgvector.sql, since now it uses raw properties column instead of _agtype_build_vertex/edge.
• Add regression tests

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Pull request overview

This pull request introduces vertex and edge as PostgreSQL composite types to optimize graph query performance. The changes enable direct field access for properties and accessor functions instead of rebuilding agtype structures.

Changes:

• Introduces vertex (id, label, properties) and edge (id, label, start_id, end_id, properties) composite types
• Optimizes property access to use direct field selection instead of agtype rebuild
• Changes _label_name to return agtype instead of cstring for type consistency
• Adds implicit casts from vertex/edge to agtype and json
• Fixes MERGE clause type mismatches with volatile wrapper

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
src/include/utils/agtype.h	Added OID accessors for VERTEXOID and EDGEOID composite types
src/include/parser/cypher_expr.h	Added helper functions for entity type checking and field extraction
src/include/parser/cypher_clause.h	Added field access helper functions for composite types
src/include/catalog/ag_label.h	Added get_label_name function signature
src/backend/utils/adt/agtype.c	Implemented composite type OID caching, vertex/edge to agtype casts, label parameter type change
src/backend/parser/cypher_expr.c	Implemented accessor function optimization and entity type coercion
src/backend/parser/cypher_clause.c	Updated entity expression builders to use RowExpr, added field selection helpers
src/backend/executor/*.c	Updated executors to use agtype labels instead of cstrings
src/backend/catalog/ag_label.c	Changed _label_name to return agtype, added cache-based get_label_name
sql/agtype_graphid.sql	Defined vertex/edge composite types and cast functions
sql/age_scalar.sql	Added _get_vertex_by_graphid helper function
sql/age_main.sql	Removed duplicate _label_name definition
regress/sql/*.sql	Added comprehensive tests for accessor optimizations and composite types

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