fix: handle unterminated Hunyuan stream chunks

[spduan78]

Summary

• parse the remaining Hunyuan stream buffer when the final SSE chunk has no trailing blank line
• ignore empty Hunyuan choice arrays instead of indexing Choices[0]
• add a wasm-host regression test for final chunks without terminators

Tests

• GOCACHE=/private/tmp/higress-go-cache go test -run TestHunyuanStreamingLastChunkWithoutTerminatorDoesNotPanic -count=1
• GOCACHE=/private/tmp/higress-go-cache go test ./provider

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