fix: parse FDv2 goodbye events with only a reason field

[keelerm84]

Summary

• The FDv2 goodbye event is specified to carry only a reason field. The Ruby SDK's Goodbye class incorrectly declared silent and catastrophe, and from_h raised ArgumentError if either was missing — meaning a spec-compliant payload would fail to parse.
• This PR removes both fields from the class, the constructor, to_h, and from_h. from_h now requires only reason.
• Streaming consumer in lib/ldclient-rb/impl/data_system/streaming.rb was logging at @logger.error and reading goodbye.silent / goodbye.catastrophe. Replaced with @logger.info { "[LDClient] SSE server received goodbye: #{goodbye.reason}" } — a goodbye is a normal protocol signal, matching the Java SDK.

Test plan

• bundle exec rspec spec/impl/data_system/streaming_synchronizer_spec.rb — 14 examples, 0 failures.
• bundle exec rspec spec/impl/data_system/ — 125 examples, 0 failures.
• grep -rn for goodbye.silent / goodbye.catastrophe — no remaining references.

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Note

Low Risk
Low risk: narrows Goodbye parsing/serialization to the spec-required reason field and adjusts logging; behavior change is limited to handling GOODBYE SSE events.

Overview
Aligns FDv2 Goodbye event handling with the protocol by removing the unsupported silent/catastrophe fields from ProtocolV2::Goodbye (constructor, to_h, and from_h now only require reason).

Updates the streaming SSE consumer to stop treating GOODBYE as an error condition and instead log it at info, and adjusts the streaming synchronizer spec to construct goodbye events with only reason.

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