Use an in-memory, shared-cache SQLite database for tests

[stefan-burke]

Summary

The test suite ran on per-worker SQLite files. Under mutant's parallel kill-forks (which all share one test file), the SQLite database lock made results non-deterministic — the Event subject flaked between 5 and 9 surviving mutations across identical runs, because a lock-timeout error during a mutation was counted as a "kill".

This switches the test environment to an in-memory, shared-cache database: file::memory:?cache=shared.

Why shared-cache

• Plain :memory: gives every connection its own private database, which breaks any test that uses two connections — notably js: true browser tests, where the test thread writes data and Capybara's Puma server thread must read it.
• cache=shared makes all connections within a process share one in-memory database, so browser tests keep working.
• Shared cache is scoped per process, so parallel_test workers and mutant kill-forks are isolated automatically. There's no file and no lock, and TEST_ENV_NUMBER is no longer needed to keep workers apart.

Changes

• config/database.yml — test now uses file::memory:?cache=shared with in-memory-appropriate pragmas (WAL/mmap dropped).
• spec/rails_helper.rb — loads db/schema.rb into the empty in-memory DB at boot (an in-memory DB starts empty every run) instead of the file-based maintain_test_schema!, and skips the parallel-worker reconnect that would drop the shared cache and its schema.
• .github/workflows/test-ruby.yml, coverage.yml — drop the now-unnecessary db:migrate / parallel:prepare steps (parallel:prepare actually errors against an in-memory DB); the schema loads at boot.
• CLAUDE.md — document the in-memory test database.

Validation (all on the in-memory DB)

• Full models, requests, services, helpers, lib, controllers, views, seeds suites: 0 failures.
• Full features suite in parallel, including a js: true browser test driving Chromium against the Puma server: 0 failures.
• parallel_rspec -n 2 across model specs: 0 failures, with no parallel:prepare.
• mutant is now deterministic: three consecutive parallel runs of Event returned an identical 9 surviving mutations (92.56%) with no database is locked errors — versus the 5↔9 flakiness on files.

🤖 Generated with Claude Code

https://claude.ai/code/session_01CLseCgvqE2jdan8K57ENJn

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