fix(plugin-postgresql): gate version-dependent system catalogs (#1240)

[datlechin]

Summary

Issue #1240: TablePro 0.40.2 fails to connect to old PostgreSQL servers with ERROR: relation "pg_matviews" does not exist. This PR introduces a PostgreSQLCapabilities model derived from PQserverVersion and routes every version-dependent SQL site through it.

Audit (the immediate bug was the tip of the iceberg)

#	Hot/Cold	Feature	Min PG	Site
1	HOT (connect)	pg_matviews	9.3	fetchTables
2	HOT (connect)	pg_foreign_table	9.1	fetchTables
3	HOT (open table)	attidentity	10	fetchColumns / fetchAllColumns / fetchTableDDL
4	HOT (open table)	attgenerated	12	same
5	WARM (indexes)	array_position()	9.5	fetchIndexes
6	WARM (DDL)	pg_sequences	9.5	fetchDependentSequences
7	COLD (create DB)	datlocprovider / daticulocale / datlocale	15 / 15 / 17	fetchTemplate1Defaults, createDatabaseFormSpec, createDatabase

Only #1 and #2 were caught by the original report — the rest were latent regressions that would have surfaced as soon as the PG < 10 user clicked on a table.

Changes

New Plugins/PostgreSQLDriverPlugin/PostgreSQLCapabilities.swift — single source of truth, each flag documents its minimum PG version inline:

struct PostgreSQLCapabilities: Sendable, Equatable {
    let serverVersion: Int32
    var hasMaterializedViewsCatalog: Bool { serverVersion >= 90_300 }
    var hasForeignTablesCatalog: Bool { serverVersion >= 90_100 }
    var hasSequencesCatalog: Bool { serverVersion >= 90_500 }
    var hasIdentityColumns: Bool { serverVersion >= 100_000 }
    var hasGeneratedColumns: Bool { serverVersion >= 120_000 }
    var hasArrayPosition: Bool { serverVersion >= 90_500 }
    var hasOrderedAggregates: Bool { serverVersion >= 90_000 }
    var hasCollationProvider: Bool { serverVersion >= 100_000 }
    var hasDatabaseICULocale: Bool { serverVersion >= 150_000 }
    var hasDatabaseLocale: Bool { serverVersion >= 170_000 }
}

LibPQPluginConnection.swift — caches the numeric value from PQserverVersion(connection) alongside the existing display string; new serverVersionNumber() -> Int32 accessor.

PostgreSQLPluginDriver.swift — adds var capabilities computed from the connection; rewrites:

• fetchTables UNION pieces are appended only when the catalog exists.
• fetchIndexes uses ORDER BY array_position(...) only when supported, falls back to ORDER BY a.attnum (deterministic ordering, may not match index logical order on PG <9.5).
• fetchTableDDL builds the identity / generated CASE clauses conditionally; the atthasdef guard adapts.
• fetchDependentSequences short-circuits when pg_sequences is unavailable (DDL still generates, just without sequence details).
• fetchTemplate1Defaults, createDatabaseFormSpec, and the CREATE DATABASE builder now consult capabilities.hasDatabaseICULocale / hasDatabaseLocale instead of the local ad-hoc parsedServerMajorVersion() (now deleted).

PostgreSQLPluginDriver+Columns.swift — fetchColumns and fetchAllColumns substitute NULL::text for attidentity / attgenerated projections when the columns don't exist, so the row shape stays stable and mapPgColumnRow needs no change.

What stays unchanged

• The pg_collation.collprovider (PG 10+) call in fetchCollations() is wrapped in do/catch that returns empty arrays, so PG <10 still falls back cleanly. Left as-is to keep the diff focused.
• Redshift driver is untouched.

Test plan

• Connect to PG ≥ 13: schema, columns, indexes, DDL, create database all work as before (no behavioral change).
• Connect to PG 9.6: schema/columns/indexes work; tables with identity/generated columns missing those markers in DDL (correct — PG 9.6 doesn't have them).
• Connect to PG 9.2: schema loads (no matviews / no foreign tables UNION); column / index queries succeed.
• Connect to PG 9.0: same; pg_sequences-dependent sequence details omitted from DDL.
• Create Database dialog: PG ≥ 15 shows libc/icu provider picker, < 15 shows only libc.

Notes

Bundled plugin — ships in the next app release. No registry republish per feedback_no_registry_for_builtins.