Add tests for Deferred log context capture and backward-compatible restore

[jsxs0]

Summary

While working on the SSE context propagation tests in #267, I traced how log context flows across fiber boundaries in the Deferred system, specifically Context.build (capture) and restore_log_info in Task (restore). I noticed two paths without test coverage:

1. Context.build log context capture

The "when rage_logger is set" context verified context[4] (log tags) but not context[5] (log context). Added a test confirming Fiber[:__rage_logger_context] is captured, and fixed the after block to clean up both fiber-locals.

2. restore_log_info backward compatibility

restore_log_info has three branches: the current Array format, a legacy string format (older versions stored request_id as a plain string at index 4), and nil. The Array and nil branches were tested, but the legacy string path, which wraps the value into an array and defaults context to {}, had no coverage.

Test plan

bundle exec rspec spec/deferred/context_spec.rb spec/deferred/task_spec.rb
58 examples, 0 failures

[rsamoilov]

Amazing!