fix: align FDv2 goodbye event with spec (reason-only)

[keelerm84]

Summary

• The FDv2 goodbye event is specified to carry only a reason field. The Python SDK's Goodbye dataclass incorrectly declared silent and catastrophe, and from_dict raised ValueError if either was missing — meaning a spec-compliant payload would fail to parse.
• This PR removes both fields from the dataclass and from to_dict/from_dict. from_dict now requires only reason.
• Streaming consumer in ldclient/impl/datasourcev2/streaming.py was logging at log.error and reading goodbye.silent / goodbye.catastrophe. Replaced with a single log.info("SSE server sent goodbye: %s", goodbye.reason) — a goodbye is a normal protocol signal, matching what the Java SDK does.

Test plan

• poetry run pytest ldclient/testing/impl/datasourcev2/ — 71 tests pass.
• poetry run pytest ldclient/testing/ — full unit test suite (1161 tests) passes.
• grep -rn "silent\|catastrophe" ldclient/ — no remaining references.

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Note

Low Risk
Low risk: change is limited to FDv2 Goodbye event serialization/parsing and corresponding log/test updates, with no new behavior beyond accepting spec-compliant payloads and adjusting log level.

Overview
Aligns the FDv2 Goodbye event with the spec by removing the silent and catastrophe fields from the Goodbye dataclass and its to_dict/from_dict methods, so parsing no longer fails on reason-only payloads.

Updates the streaming SSE consumer to treat GOODBYE as a normal protocol signal (logs info with the reason only) and adjusts datasourcev2 polling/streaming tests to construct Goodbye with just reason.

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