Let a retry policy decide will_be_retried in the runner

[Enceradeira]

Description

Alternative to #344, fixing cucumber/cucumber-ruby#1905 together with cucumber/cucumber-ruby#1910.

Since 19.0.0 the Runner emits the TestCaseFinished envelope and decides its will_be_retried field from a maximum number of attempts. The runner cannot know that on its own: whether a test case runs again is decided by whoever runs it again, which in cucumber-ruby is the retry filter honouring both --retry and --retry-total. #344 copies the --retry-total counter into the runner, which makes the reported symptom go away but keeps the retry decision in two places, and leaves two further defects in the message stream:

• cucumber-ruby passes the number of retries where the runner expects the number of attempts. With --retry 1 the flag is never true, so a retried attempt is reported as a final result and the HTML report shows the scenario twice. With --retry 2 the second attempt is reported as final although a third one follows.
• The runner broadcasts the test_case_finished event before emitting the TestCaseFinished envelope. The retry filter re-runs the test case from inside that event, so the envelope of the first attempt is emitted after the retry and carries the test_case_started_id of the retry. On main, a retried scenario produces two TestCaseFinished envelopes for attempt 2 and none for attempt 1.

The compatibility kit does not catch any of this because it only compares message keys, not values.

This PR:

• replaces the max_attempts constructor argument with a retry_policy, an object answering will_be_retried?(test_case, result). With no policy, no test case is reported as retried.
• emits the TestCaseFinished envelope before broadcasting the test_case_finished event, so a retry started from that event no longer reorders the stream.
• adds specs for the envelope values and for the re-entrant retry case, which previously had no coverage.

Type of change

• Bug fix
• Breaking change (constructor argument of Cucumber::Core::Test::Runner), see upgrading_notes/20.0.0.md

Checklist:

• Tests have been added for any changes to behaviour of the code
• New and existing tests are passing locally and on CI
• bundle exec rubocop reports no offenses
• RDoc comments have been updated
• CHANGELOG.md has been updated