[Bug]: RustToolParser streaming can drop buffered text at stream end

## Your current environment

- vLLM `main` at `04d13b5`
- Reproduced with the Rust tool-parser Python extension in the `vllm/vllm-openai:latest` container
- CPU-only parser reproduction; no model weights or GPU execution are required

## Describe the bug

`RustToolParser` does not finalize the underlying Rust parser when streaming generation ends. A Rust parser may retain an ambiguous partial tool-call marker in its incremental buffer, so the buffered suffix can be silently omitted from streamed content.

For MiniMax M3, the minimal model output is:

```text
plain ]<]minimax[>[[

Start with the streaming adapter's TODO about calling Rust finish(), then trace how DelegatingParser reports finished=True and compare it with the non-streaming adapter's parse_into and finish sequence. Reproduce the CPU-only MiniMax M3 string and verify that stream-end output preserves the buffered suffix, including recovery through reset() when finalization fails for incomplete syntax.

Written by the indexing model from the issue text.