Raise an actionable error when a cache predates a coder

[navidemad]

Summary

MemoryCache#data_for did a bare data.fetch(coder_class), so when the on-disk cache had no entry for a registered coder it surfaced as an opaque KeyError: key not found: FixtureKit::SomeCoder, with no hint about the cause or the fix.

This happens when the consuming process has a coder registered that wasn't registered when the cache was written. The common case is FIXTURE_KIT_PRESERVE_CACHE=1: a cache from a previous run gets read by a process that has since added a coder. The lookup runs from both Cache#load (mount) and Cache#evaluate (parent data), so either path can hit it.

Change

Replace the bare fetch with a fetch block that raises a new FixtureKit::MissingCoderDataError, naming the coder and pointing at cache regeneration:

The fixture cache has no data for FixtureKit::SomeCoder. This usually means
the cache was written before FixtureKit::SomeCoder was registered as a coder.
Regenerate the fixture cache so it includes this coder (clear the cache
directory, or run without FIXTURE_KIT_PRESERVE_CACHE=1).

Same fail-loud, stale-cache-hint approach the foreign-key check in ActiveRecordCoder#mount already uses.

Test plan

• bundle exec rspec spec/unit/memory_cache_spec.rb
• bundle exec rspec (full suite)