[FEATURE] Treat disabled flag evaluation as success with reason=DISABLED

[suthar26]

Requirements

flagd currently treats disabled flags as errors (reason=ERROR, errorCode=FLAG_DISABLED). This should instead resolve successfully with reason=DISABLED, returning the flag's defaultVariant value.

Expected behavior:

• Disabled flags resolve successfully across gRPC, OFREP, and in-process evaluation
• Missing flags still return FLAG_NOT_FOUND
• ResolveAll includes disabled flags instead of omitting them
• Telemetry records disabled evaluations as non-errors
• This would align flagd with the OpenFeature resolution reason model and avoid conflating intentionally disabled flags with missing flags.