Expand file_fixture_path against Rails.root

[hammadxcm]

Fixes #2682.

Problem

The configured default file_fixture_path is the relative string 'spec/fixtures/files' (lib/rspec/rails/configuration.rb:84). When applied to example groups via FileFixtureSupport, it stays relative. But ActiveSupport::TestCase (and downstream consumers such as ActionMailer's read_fixture) treat file_fixture_path as absolute — they hand it directly to Pathname.new / File.join expecting a rooted path.

Issue reporter @jasonkarns hit this while fixing read_fixture and noted that all Rails-side defaults for file_fixture_path are absolute (either via File.expand_path(__dir__) or Rails.root.join(...)).

Fix

Expand the configured value against Rails.root in FileFixtureSupport.included before assigning it to the example group and to ActiveStorage::FixtureSet. File.expand_path is a no-op on already-absolute paths, so users who override the config with an absolute path see no behavioural change.

The user-facing config default (config.file_fixture_path = 'spec/fixtures/files') is intentionally left as the relative form — that's the conventional value people write in RSpec.configure and matches what documentation references. Only the value that gets exposed to example groups becomes absolute.

Tests

New spec spec/rspec/rails/file_fixture_support_spec.rb covers three cases:

1. A relative configured path gets expanded against Rails.root.
2. An absolute configured path passes through unchanged.
3. ActiveStorage::FixtureSet.file_fixture_path receives the expanded value.

Test plan

• bundle exec rspec spec/rspec/rails/file_fixture_support_spec.rb — 3/3 pass
• bundle exec rspec spec/rspec/rails/configuration_spec.rb spec/rspec/rails/fixture_support_spec.rb — 67/67 pass
• bundle exec rubocop on changed files — clean