refactor: move oid name-lookup machinery out of the coercion rule

[sunng87]

Problem

The oid-coercion analyzer rule (oid_coercion_rule.rs) builds a TableProvider-backed LogicalPlan subquery inline to resolve name strings to oids (e.g. relnamespace = 'public' → (SELECT oid FROM pg_namespace WHERE nspname = 'public')). Depending on TableProvider and LogicalPlanBuilder is out of scope for an analyzer rule, which should be a pure expression rewriter.

Change

Move all data-access concerns behind the OidLookupProvider trait:

• Trait method changes from
  build_table_provider(name) -> Option<Arc<dyn TableProvider>>
  to
  resolve_name_expr(kind, name) -> Option<Expr> — the rule asks for a resolved oid-valued expression and never sees a TableProvider, a LogicalPlanBuilder, or the kind→table mapping.
• The kind→table mapping (oid_lookup_target) and the subquery construction (build_oid_name_subquery) move to pg_catalog.rs, next to the provider that already owns build_table_by_name — the natural home for catalog data access.
• resolve_operand in the rule collapses to: numeric string → literal Int32 (inline, no catalog); name string → provider.resolve_name_expr.

The rule file drops its imports of TableProvider, provider_as_source, LogicalPlanBuilder, Subquery, and Spans, and shrinks by ~54 net lines.

Why the subquery mechanism stays

Name resolution for dynamic tables (pg_namespace, pg_class) must stay lazy: the oids for those tables are assigned during execution, not at plan time, so the resolution cannot be collapsed to an eager lookup. The scalar-subquery approach is still used — it just now lives in the provider rather than the rule. The rule's goal ("support Postgres's hidden coercion from string to oid") is unchanged; only where the lookup is implemented moves.

Test plan

Behavior is unchanged — the same lazy scalar subquery is produced.

• cargo test --workspace — 47 pg-catalog lib tests + 8 client integration tests pass, including every query that depends on name resolution:
  • dbeaver: c.relnamespace = 'public'
  • metabase: classoid = 'pg_class'::regclass
  • pgcli: prorettype::regtype != 'trigger'::regtype
  • pgadbc: cls.oid = 'clubs'::regclass::oid
• cargo clippy -p datafusion-pg-catalog --lib — no new warnings
• cargo fmt --all --check — clean
• moved lookup_target unit test → oid_lookup_target_maps_kinds_to_backing_tables in pg_catalog.rs