feat: Populate OpenFeature flag metadata from the evaluation reason

[kinyoklion]

Populates OpenFeature flag metadata from the LaunchDarkly evaluation reason so consumers (and the OpenFeature/OTel telemetry hooks) can read experiment, rule, prerequisite, and Big Segments information.

• Adds flat, camelCase keys variationIndex, inExperiment, ruleIndex, ruleId, prerequisiteKey, bigSegmentsStatus; each is omitted when it does not apply.
• Applies to the shared server translator, so it covers the Node provider (and any future server provider built on it).
• Matches the behavior now specified in sdk-specs#253 and implemented in the Java, .NET, Python, and Ruby providers.
• Documents the keys in the Node provider README.

@cursor review

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

Implementation details

translateResult now builds a FlagMetadata object from LDEvaluationDetail:

• variationIndex is included only when the SDK returned a variation, so default/error evaluations omit it.
• inExperiment is emitted only when true, rather than emitting false for every non-experiment evaluation.
• ruleIndex/ruleId, prerequisiteKey, and bigSegmentsStatus are copied through when the reason provides them.

Key names are named constants so they cannot drift from the spec.

Backend-only change; no UI, so no screenshots or staging preview apply.

Testing: yarn workspace @launchdarkly/openfeature-js-server-common test and ... lint, plus yarn workspace @launchdarkly/openfeature-node-server test. The existing BaseOpenFeatureProvider result assertions were updated for the added flagMetadata field.

Describe alternatives you've considered

Emitting the full reason object under a single nested key was rejected — OpenFeature flag metadata values must be scalars, and flat keys are what the spec draft standardizes across providers.

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

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[github-actions]

@launchdarkly/js-sdk-common size report
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@launchdarkly/browser size report
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@launchdarkly/js-client-sdk size report
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@launchdarkly/js-client-sdk-common size report
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Uncompressed size: 224541 bytes