fix: Support non-public PostgreSQL schemas in resource generator

[elliotb]

This PR fixes the resource generator to properly handle tables in non-public PostgreSQL schemas (e.g., inventory.products, ocloud.roles). Previously, the generator would fail when attempting to introspect tables outside the public schema. e.g.:

mix ash_postgres.gen.resources OgatApp.AshDomains.GrantedRoles --tables ocloud.roles --snapshots-only
Generating resources from [OgatApp.Repo]
** (Postgrex.Error) ERROR 42P01 (undefined_table) relation "roles" does not exist
    (ecto_sql 3.13.2) lib/ecto/adapters/sql.ex:1098: Ecto.Adapters.SQL.raise_sql_call_error/1
    (ash_postgres 2.6.20) lib/resource_generator/spec.ex:127: AshPostgres.ResourceGenerator.Spec.add_foreign_keys/1
    (ash_postgres 2.6.20) lib/resource_generator/spec.ex:113: anonymous fn/1 in AshPostgres.ResourceGenerator.Spec.tables/2
    (elixir 1.18.0) lib/enum.ex:1714: Enum."-map/2-lists^map/1-1-"/2
    (ash_postgres 2.6.20) lib/resource_generator/spec.ex:111: anonymous fn/2 in AshPostgres.ResourceGenerator.Spec.tables/2
    (ecto_sql 3.13.2) lib/ecto/migrator.ex:170: Ecto.Migrator.with_repo/3
    (ash_postgres 2.6.20) lib/resource_generator/spec.ex:86: AshPostgres.ResourceGenerator.Spec.tables/2
    (elixir 1.18.0) lib/enum.ex:4438: Enum.flat_map_list/2

Two issues have been fixed:

1. When generating resources from tables like ocloud.roles, PostgreSQL queries would fail with
   ERROR 42P01 (undefined_table) relation "roles" does not exist because unqualified table names were used in introspection queries.

2. After generating resources, the migration generator would crash with could not list directory "priv/resource_snapshots/repo/roles": no such file or directory because it tried to ls snapshot directories based on unqualified table names.

The following changes were made:

• Use qualified table names (schema.table) in all PostgreSQL introspection queries for foreign keys, check constraints, and indexes
• Add schema option to postgres configuration for generated resources when schema is not "public"
• Guard against missing snapshot directories to handle new resources and schema-prefixed paths gracefully
• Added test coverage for the resource generator for non-public schema support with foreign keys and indexes

Contributor checklist

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• I accept the AI Policy, or AI was not used in the creation of this PR. (Claude Code assisted in the development of this PR)
• Bug fixes include regression tests
• Chores
• Documentation changes
• Features include unit/acceptance tests
• Refactoring
• Update dependencies

[zachdaniel]

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