v0.27.0: --preload auto-detect hard-fails when only spec/spec_helper.rb exists

[marinazzio]

Context

Tried evilution v0.27.0 on a Rails 7 app using RSpec. The repo only has
spec/spec_helper.rb — no spec/rails_helper.rb (Rails not eagerly loaded
for unit specs is a deliberate convention).

Friction: preload auto-detect doesn't fall through

$ bundle exec evilution app/models/.../foo.rb --jobs 4 --save-session --format text --no-progress
Error: preload file not found: "spec/rails_helper.rb"

--help says auto-detect order is:

auto-detect spec/rails_helper.rb -> spec/spec_helper.rb -> test/test_helper.rb

But the actual behavior hard-errors on the first miss instead of trying
the next candidate. Workaround: --preload spec/spec_helper.rb on every
invocation.

Suggested fix

When the auto-detected candidate is absent, silently fall through to the
next one. Only emit preload file not found if NONE of the three exist.

This would match the help text and cover Rails projects that ship only
spec_helper.rb.

Other observations (minor)

• Datadog log noise: hundreds of WARN -- datadog: Failed to convert tag: tag 'k8s_pod:' ends with a colon lines per run. Drowns useful output.
  Maybe suppress non-error datadog logs when --no-progress is set, or
  document an env-var to silence.

• Rails-load overhead per file: ~30s per invocation to boot Rails. 5 files
  → ~2.5 min spent on bootstrap before any mutation runs. Multi-file batch
  mode (evilution file1 file2 ... amortising the parent process) would
  flatten this.