feat: Populate OpenFeature flag metadata from the evaluation reason

[kinyoklion]

Populates OpenFeature flag metadata with the LaunchDarkly specific parts of the evaluation result, so hooks (including the OpenTelemetry hook) can read them.

• Keys mirror the evaluation reason fields: variationIndex, inExperiment, ruleIndex, ruleId, prerequisiteKey, bigSegmentsStatus.
• Each key is omitted rather than default-valued when it does not apply, so a consumer can distinguish "not in an experiment" from "no information".
• Matches openfeature-java-server#56, openfeature-dotnet-server#61 and openfeature-python-server#53; the naming is being defined in an sdk-specs spec.

No screenshots or staging preview apply - this is a server-side library change with no UI.

Implementation details

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Describe the solution you've provided

ResolutionDetailsConverter#to_resolution_details now passes flag_metadata:

flag_metadata: flag_metadata(reason, is_default ? nil : variation_index)

Each entry is gated on the corresponding reason field being present rather than on the reason kind, so no key is written from a sentinel value. big_segments_status is a symbol in the Ruby SDK, so it is stringified to keep the metadata value types consistent with the other providers.

The README gains a "Flag Metadata" section documenting the keys.

Describe alternatives you've considered

Exposing the reason object itself as one metadata entry would be closer to the LaunchDarkly shape, but OpenFeature flag metadata values are scalars, so the flat keys are the only available shape.

Additional context

docker run --rm -v "$PWD":/app -w /app ruby:3.4 bash -c "bundle install && bundle exec rake"

61 examples, 0 failures; RuboCop clean. 7 new examples cover each key and the omissions.

@cursor review

Link to Devin session: https://app.devin.ai/sessions/0c452d209ec54b068ba120b4c92b8f6c
Requested by: @kinyoklion

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