Fix ArgumentError from TestFixtures setup_shared_connection_pool

[mnovelo]

Summary

• Fix ArgumentError: pool_config for :reading role and :apartment_...:reading shard was nil caused by Rails' setup_shared_connection_pool iterating apartment's role-specific tenant shards
• Auto-wire cleanup via Railtie on_load(:active_record_fixtures) hook, matching activerecord-tenanted's precedent for fixture integration
• Guard ivar prevents re-entry from the !connection.active_record notification subscriber mid-example

Problem

Rails' ActiveRecord::TestFixtures#setup_shared_connection_pool assumes every shard has a :writing pool_config. Apartment v4 registers tenant pools under role-specific custom shard keys (e.g., shard :apartment_acme:reading under role :reading). When fixture setup encounters this shard, writing_pool_config resolves to nil, and set_pool_config(role, shard, nil) raises ArgumentError.

Surfaces when a before(:all) block (or prior test) triggers the elevator under connected_to(role: :reading), creating a tenant pool, and a subsequent example's fixture setup discovers and iterates it.

Changes

File	What
lib/apartment/test_fixtures.rb	New Apartment::TestFixtures module: overrides setup_shared_connection_pool to deregister apartment pools before Rails iterates them
lib/apartment/railtie.rb	on_load(:active_record_fixtures) hook, guarded by Rails.env.test? and config.test_fixture_cleanup
lib/apartment/config.rb	test_fixture_cleanup attribute (default true) — escape hatch to disable
spec/unit/test_fixtures_spec.rb	6 specs: regression test, cleanup, pool manager clear, re-entry guard, teardown reset
docs/designs/v4-test-fixtures-compatibility.md	Design spec
docs/plans/apartment-v4/test-fixtures-compatibility.md	Implementation plan

Test plan

• bundle exec appraisal rails-8.1-sqlite3 rspec spec/unit/test_fixtures_spec.rb — 6 examples, 0 failures
• bundle exec appraisal rails-8.0-sqlite3 rspec spec/unit/test_fixtures_spec.rb — 6 examples, 0 failures
• bundle exec appraisal rails-8.1-sqlite3 rspec spec/unit/ — 688 examples, 0 failures
• bundle exec rubocop on all changed files — no offenses

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