fix: NPE in streaming chat-completions function chain #1963

[astsiapanay]

Fixes a null-pointer exception in the streaming /chat/completions response-function chain that silently fires on nearly every streaming chunk with no attachments, logged as "Function call is failed with error. Try to recover SSE event" and swallowed by generic error recovery.

Applicable issues

• fixes NPE in streaming chat-completions function chain: "Function call is failed with error. Try to recover SSE event" #1963

Description of changes

• CollectResponseAttachmentsFn.apply() now returns Future.succeededFuture(tree) instead of a null-valued future when a chunk has no attachments, so the tree is passed through unchanged instead of dropped.
• CollectChatCompletionUsageFn.apply() now guards against a null tree input, preventing a similar mistake elsewhere in the chain from causing the same regression.
• BufferingReadStream.BaseEventListener.handle() now guards against a null/blank SSE data payload before parsing it as JSON.
• Added a regression test (CollectResponseChatCompletionAttachmentsFnTest#testApplyPassesTreeThroughUnchangedWhenNoAttachments) and a null-tolerance test (CollectChatCompletionUsageFnTest#testTolerantOfNullTree), plus a new ChatCompletionFunctionChainTest that exercises the real 3-function chain used by DeploymentPostController end-to-end, closing the test gap that let this regression through originally (each function was previously only tested in isolation).

Checklist

• Title of the pull request follows Conventional Commits specification

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

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