feat: declarative config_overrides for FixtureLoader

[erikfrerejean]

Summary

Adds a declarative config_overrides map to FixtureLoader so consuming modules can force config statics on specific classes during a fixture write — without hand-rolling a bespoke onBeforeLoad extension.

This targets the common case where a model has a write-time side effect keyed on a config static (auto-scaffolding, auto-publishing, denormalisation hooks) that must be suppressed while top-down fixtures are loaded. Previously each consumer wrote an Extension subclass; now it's YAML:

WeDevelop\E2e\Fixtures\FixtureLoader:
  config_overrides:
    My\Module\Model\Section:
      auto_scaffold: false
    My\Module\Model\Row:
      auto_scaffold: false

How it works

Each override is applied from a FixtureBlueprint beforeCreate callback. FixtureBlueprint::createObject() invokes that callback inside its own Config::nest()/unnest() window, so the value is live only for that record's write and reverts immediately afterwards — it never leaks into normal app code. Overrides are applied before the onBeforeLoad hook, so a consumer's dynamic extension can still override the same class (a later FixtureFactory::define() wins).

The onBeforeLoad hook remains for anything a static value can't express (dynamic values, non-config side effects).

Design rationale

This is configuration (which statics, what values), not behaviour — so expressing it as YAML rather than a PHP Extension is the more honest abstraction. It does not remove any flexibility: the hook is still there for the dynamic cases.

Tests

Four integration tests on FixtureLoaderTest, covering the full contract:

• override reaches the record at write time
• override does not leak past the load (config reverts)
• multiple classes + multiple keys per class all apply
• absent config_overrides is a no-op (default [])

Backed by two support models: E2eScaffoldingObject (proves real suppression behaviour) and E2eConfigProbeObject (observes the override at write time and proves no leak).

Verification

• Full PHP suite green (59 tests, 144 assertions)
• PHPStan clean on both the 8.3 and 8.5 configs
• Rector clean

Consumer migration (separate repo)

A consuming module can now delete its bespoke suppression Extension + its extensions: registration and replace both with the config_overrides block above. Not included here — that's a change in the consuming repo.