[Rust Frontend] Recover buffered text from incomplete tool calls at EOS

[reidliu41]

Purpose

When a Gemma4 tool-call stream is truncated before the closing marker, Gemma4UnifiedParser::finish() returns an incomplete-tool-call error while the raw tool-call text remains buffered inside the parser. The unified stream wrapper previously closed parser state without recovering that buffer, so clients could receive only Done and an empty assistant message.

This PR mirrors the existing mid-stream parse-error fallback for end-of-stream finish errors: reset the parser, emit the recovered buffered text as a plain text delta, and close any open tool-call state. It also adds coverage for both the generic finish-error path and a real Gemma4 truncated tool-call stream.

Test Plan

Test Result

cargo test -p vllm-chat unified_stream_finish_error_recovers_buffered_text -- --nocapture
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.17s
     Running unittests src/lib.rs (target/debug/deps/vllm_chat-59266da177b74ae3)

running 1 test
test output::default::unified::tests::unified_stream_finish_error_recovers_buffered_text ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 179 filtered out; finished in 0.00s

     Running tests/chat.rs (target/debug/deps/chat-cd39677c8b0b7af8)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 17 filtered out; finished in 0.00s

     Running tests/roundtrip.rs (target/debug/deps/roundtrip-097e04ca5d9e4d96)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 15 filtered out; finished in 0.00s

cargo test -p vllm-chat unified_stream_recovers_incomplete_gemma4_tool_call_at_eos -- --nocapture
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.10s
     Running unittests src/lib.rs (target/debug/deps/vllm_chat-59266da177b74ae3)

running 1 test
test output::default::unified::tests::unified_stream_recovers_incomplete_gemma4_tool_call_at_eos ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 179 filtered out; finished in 0.00s

     Running tests/chat.rs (target/debug/deps/chat-cd39677c8b0b7af8)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 17 filtered out; finished in 0.00s

     Running tests/roundtrip.rs (target/debug/deps/roundtrip-097e04ca5d9e4d96)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 15 filtered out; finished in 0.00s

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[BugenZhao]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. What shall we delve into next?

Reviewed commit: a15883a650

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[BugenZhao]

Thanks!