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 names of 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; 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

EvaluationDetailExtensions gains a ToFlagMetadata extension on EvaluationReason, and both the success and error paths of ToResolutionDetails pass its result as flagMetadata:

private static ImmutableMetadata ToFlagMetadata(this EvaluationReason reason, int? variationIndex)
{
    var metadata = new Dictionary<string, object>();
    if (variationIndex.HasValue) metadata[VariationIndexKey] = variationIndex.Value;
    if (reason.InExperiment) metadata[InExperimentKey] = true;
    if (reason.RuleIndex.HasValue) metadata[RuleIndexKey] = reason.RuleIndex.Value;
    // ...
    return new ImmutableMetadata(metadata);
}

ruleIndex and ruleId are gated on the reason actually carrying them rather than on the reason kind, so no key is written from a sentinel value.

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

Describe alternatives you've considered

A single nested reason structure would be closer to the LaunchDarkly evaluation reason, but ImmutableMetadata only holds scalar values, so the flat keys are the only shape available.

Testing

BUILDFRAMEWORKS=netstandard2.0 dotnet build src/LaunchDarkly.OpenFeature.ServerProvider -f netstandard2.0
BUILDFRAMEWORKS=net8.0 dotnet test test/LaunchDarkly.OpenFeature.ServerProvider.Tests -f net8.0

All tests pass, including 8 new cases covering each key, the omissions, and structure evaluations.

Additional context

net471 could not be exercised locally because Mono is not installed on the build machine; CI covers it.

@cursor review

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

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