feat: support schema-qualified table names for multi-schema databases

[Daniel230196]

Summary

Adds support for schema-qualified table names (e.g. shop.orders) in pgxgen.yaml, enabling CRUD, models, constants, and sqlc config generation for tables in non-default PostgreSQL schemas.

Motivation

PostgreSQL supports organizing objects into schemas — a common pattern for multi-tenant apps, domain separation, and large codebases. Previously,
pgxgen only worked with tables in the default public schema. Users with multi-schema databases had to either:

• Maintain separate configs per schema
• Manually write SQL for non-public tables
• Avoid using schemas altogether

This PR brings pgxgen's behavior in line with sqlc's schema-qualified naming convention, where objects in non-default schemas are prefixed with the schema
name (e.g. shop.orders → Go type ShopOrder).

Usage

tables:
  users:              # public schema — no prefix needed
    primary_column: id
    crud:
      methods:
        get: {}

  shop.orders:        # "shop" schema — dot notation
    primary_column: id
    crud:
      methods:
        get: {}
        create: { returning: "*" }

┌───────────────────┬───────────────────────┬───────────────────────────────┐
│      Aspect       │    orders (public)    │   shop.orders (non-default)   │
├───────────────────┼───────────────────────┼───────────────────────────────┤
│ SQL               │ SELECT * FROM orders  │ SELECT * FROM shop.orders     │
├───────────────────┼───────────────────────┼───────────────────────────────┤
│ Go identifiers    │ CreateOrder, GetOrder │ CreateShopOrder, GetShopOrder │
├───────────────────┼───────────────────────┼───────────────────────────────┤
│ File paths        │ sql/queries/orders/   │ sql/queries/shop_orders/      │
├───────────────────┼───────────────────────┼───────────────────────────────┤
│ Go types (models) │ Order                 │ ShopOrder                     │
├───────────────────┼───────────────────────┼───────────────────────────────┤
│ Enum types        │ OrderStatus           │ ShopOrderStatus               │
└───────────────────┴───────────────────────┴───────────────────────────────┘

Backward compatibility

- No breaking changes — plain table keys (users, orders) behave exactly as before
- Tables in the default schema (public for PostgreSQL, main for SQLite) are never prefixed
- The DefaultSchema is derived from the catalog (not hardcoded), so engine-specific defaults are respected
- All existing tests pass unchanged

Key design decisions

1. Single ParseTableKey() entry point — all generators (CRUD, constants, models, sqlc config) use the same TableKeyParts struct, ensuring consistent naming across the entire pipeline
2. SQL name vs Go name separation — SQLTableName() returns shop.orders for SQL statements, GoName() returns shop_orders for file paths and Go identifiers
3. Two-pass enum matching — schema-qualified column types (e.g. shop.order_status) first match by exact schema+name, then fall back to bare name matching for backward compatibility
4. Matches sqlc convention — generated Go type names follow https://docs.sqlc.dev/en/latest/reference/config.html of prefixing non-default schema objects