Fix Rails edge compatibility (Kimi K3)

[amedeo]

Problem

On Rails edge (8.2.0.alpha), the DDL and counter-cache specs fail with PG::NotNullViolation: the temporal view's INSTEAD OF trigger receives NULL for omitted/defaulted columns (see #414).

Root cause

Rails 8.2 refactored schema introspection: columns(table) no longer calls column_definitions(table_name). It now uses a new private bulk API, fetch_column_definitions(tables), which resolves relations through current_schemas(false) (the schema search path) instead of casting names to regclass.

ChronoModel redirects temporal tables to the temporal schema by overriding column_definitions — and that override is silently bypassed. Column metadata is then read from the public view, which carries no column defaults and no NOT NULL constraints, instead of from the temporal table. Consequently chrono_public_view_ddl never executes its ALTER VIEW ... ALTER COLUMN ... SET DEFAULT statements, so:

• the INSTEAD OF INSERT trigger receives NULL for omitted/defaulted columns → PG::NotNullViolation in the DDL specs;
• models lose their DB-level attribute defaults (e.g. articles_count read as nil instead of 0 in the counter-cache spec).

Note that the primary_key, indexes and default_sequence_name overrides are not affected: on edge they remain the public entry points and delegate to the bulk fetchers internally, under ChronoModel's on_schema(TEMPORAL_SCHEMA) wrapping.

Fix

lib/chrono_model/adapter.rb — adds a fetch_column_definitions override next to the existing column_definitions override, feature-guarded with PostgreSQLAdapter.private_method_defined?(:fetch_column_definitions) (inert on Rails ≤ 8.1). It partitions chrono vs. plain tables and reads chrono tables inside on_schema("#{TEMPORAL_SCHEMA},#{schema_search_path}", recurse: :ignore), mirroring the existing override. Mixed bulk calls (e.g. schema cache add_all calling columns(all_tables)) are handled correctly.

The on_schema spec coupling to the schema_search_path= behavior change on PostgreSQL 18+ was resolved separately on master by #425 and #427, so this PR is now production-code only.

Test results

Full suite run locally (Ruby 3.4) against this branch rebased on master, with per-gemfile regenerated dummy test apps:

Configuration	Result
rails_edge + PostgreSQL 18.6	✅ 509 examples, 0 failures
rails_edge + PostgreSQL 16.11	✅ 509 examples, 0 failures
rails_8.1 + PostgreSQL 18.6	✅ 509 examples, 0 failures
rails_8.0 + PostgreSQL 18.6	✅ 509 examples, 0 failures
rails_7.2 + PostgreSQL 18.6	✅ 509 examples, 0 failures
rails_7.1 + PostgreSQL 18.6	✅ 509 examples, 0 failures
rails_7.0 + PostgreSQL 18.6	✅ 511 examples, 0 failures
RuboCop (87 files)	✅ no offenses

On master without this patch, the Rails edge jobs fail with the 7 remaining root-cause examples (6 in ddl_spec, 1 in counter_cache_race_spec); this branch fixes all of them.

Fixes #414