feat: Add goodbye-event FDv1 fallback directive test (SDK-2511)

[devin-ai-integration]

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

Related issues

SDK-2511 — Strengthen FDv1 fallback contract test to validate flag delivery

Describe the solution you've provided

The existing FDv1FallbackDirective tests exercise the X-LD-FD-Fallback HTTP response header form of the directive. This PR adds coverage for the in-band goodbye SSE event form (protocolFallbackTTL field on the goodbye event), per FDV2PL spec §3.7 and CSFDV2 spec §8.3.4.

This transport is essential for browser SDKs using native EventSource, which cannot read HTTP response headers on streaming connections.

Changes:

• framework/types.go: Add ProtocolFallbackTTL *int field to Goodbye struct (matches goodbye.json schema)
• mockld/streaming_service.go: Add PushGoodbyeWithFallback(reason, ttlSeconds) helper
• sdktests/common_tests_stream_fdv2.go: Add DirectiveViaGoodbyeEngagesFDv1 test — delivers a full FDv2 streaming basis, pushes a goodbye with protocolFallbackTTL: 0, then asserts the SDK transitions to FDv1 and evaluates flag values from FDv1 data

Describe alternatives you've considered

Could have added the protocolFallbackTTL field directly to PushGoodbye, but a separate method keeps the existing goodbye-without-fallback call sites clean and makes the intent explicit.

Additional context

The unparam lint warning on fdv1FallbackBody's flagKey parameter (all callers pass "flag-key") was suppressed with a //nolint directive since the parameter exists for API flexibility.

Link to Devin session: https://app.devin.ai/sessions/2d369b0a4d114c6d9fd948cc39f26a9c

[devin-ai-integration]

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[devin-ai-integration]

@cursor review

[kinyoklion]

I am a little concerned that the dropped connection and the SDK processing the fallback might race?

[devin-ai-integration]

Good catch — removed conn.Cancel() entirely. The test now waits for the SDK to close the streaming connection itself (via conn.Context.Done()), which proves the SDK actually processed the goodbye event before transitioning. Force-cancelling could have raced the SDK's event processing and masked bugs where the goodbye is ignored.