fix(FN-2013): flush buffered SSE events at stream end

- Remove the trailing-newline guard when flushing pending SSE buffer data in streamChatResponse
- Re-enable the regression test covering final done events without a trailing blank line
- Add parser coverage for chunk-split events, newline-terminated streams, and incomplete trailing fragments
- Add a patch changeset for @gsxdsm/fusion documenting the SSE final-buffer flush fix

.changeset/fix-sse-final-buffer-flush.md

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+---
+"@gsxdsm/fusion": patch
+---
+
+Fix dashboard `streamChatResponse()` SSE parsing so buffered final events are flushed when streams end without a trailing newline, and add regression coverage.