fix(tool): preserve non-streamed variables in response answers

[ZhXZhao]

Important

The contribution guidelines and existing open/closed issues and PRs were reviewed. This fixes the upstream cause of the Dify report below rather than adding a host-side workaround.

Related Issue

Fixes langgenius/dify#44141

Summary

Non-streaming custom tool variables are stored in node outputs but were also given empty stream-finalization events. ResponseStreamFilter treated those selectors as closed and skipped its variable-pool fallback, so an Answer node could have correct final graph output while its streamed/persisted reply omitted the variable value.

Track which custom variables actually emitted stream chunks, and emit final markers only for those variables. Keep standard text stream finalization and existing variable assignment/output types unchanged, including repeated assignments and empty streamed values.

Add regression tests covering:

• real Start -> session Tool -> text Tool -> Answer execution through Engine and ResponseStreamFilter;
• equality between the filtered answer and final graph output for static/streamed and empty/non-empty session IDs;
• non-streaming output types and repeated-assignment semantics.

Validation

• 50 tool-node and response-stream-filter tests passed.
• The new tests fail against the original ToolNode loaded in memory: 9 expected failures, including correct final graph output but a truncated filtered reply.
• Full suite: 994 passed on Python 3.12.10 with two workers. An initial EPUB-test NLP-model download failure was resolved by installing the pinned, SHA-256-verified en_core_web_sm model; the complete suite then passed.
• Full Ruff formatting/lint, ty type checking, lockfile validation, and architectural import checks passed.
• Independent test, knowledge, and final naming reviews completed; no documentation drift was found.
• Also verified the focused backport against graphon 0.7.0 with a real in-memory engine. No deployed source, workflow data, or credentials were modified.

Dify PR langgenius/dify#43986 applies the same empty-stream guard to Agent V2, but does not change ToolNode. This PR fixes the corresponding tool-node case described in langgenius/dify#44141.

Checklist

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[ZhXZhao]

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