Add pg_upgrade support functions

[jrgemignani]

Add pg_upgrade support functions for PostgreSQL major version upgrades

NOTE: This PR was created with AI tools and a human.

The ag_graph.namespace column uses the regnamespace type, which pg_upgrade cannot handle in user tables. This commit adds four SQL functions to enable seamless PostgreSQL major version upgrades while preserving all graph data.

New functions in ag_catalog:

• age_prepare_pg_upgrade(): Converts namespace from regnamespace to oid, creates backup table with graph-to-namespace mappings
• age_finish_pg_upgrade(): Remaps stale OIDs after upgrade, restores regnamespace type, invalidates AGE caches
• age_revert_pg_upgrade_changes(): Cancels preparation if upgrade is aborted
• age_pg_upgrade_status(): Returns current upgrade readiness status

Usage:

1. Before pg_upgrade: SELECT age_prepare_pg_upgrade();
2. Run pg_upgrade as normal
3. After pg_upgrade: SELECT age_finish_pg_upgrade();

The functions include automatic cache invalidation by touching graph namespaces, ensuring cypher queries work immediately without requiring a session reconnect.

Files changed:

• sql/age_pg_upgrade.sql: New file with function implementations
• sql/sql_files: Added age_pg_upgrade entry
• age--1.7.0--y.y.y.sql: Added functions for extension upgrades

All regressions tests passed.

[Copilot]

Pull request overview

Adds PostgreSQL pg_upgrade helper functions to Apache AGE to work around regnamespace being unsupported by pg_upgrade in user tables, enabling major-version upgrades while preserving graph metadata.

Changes:

• Introduces age_prepare_pg_upgrade(), age_finish_pg_upgrade(), age_revert_pg_upgrade_changes(), and age_pg_upgrade_status() in ag_catalog.
• Registers the new SQL file in the extension build/install process.
• Adds the same functions to the extension upgrade script for 1.7.0 -> y.y.y.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 17 comments.

File	Description
sql/sql_files	Adds the new age_pg_upgrade SQL script to the extension SQL composition list.
sql/age_pg_upgrade.sql	Implements the four pg_upgrade support functions (prepare/finish/revert/status).
age--1.7.0--y.y.y.sql	Adds the same functions to the extension upgrade path script.

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