GRPOTrainer/async: fix prefix EOS slicing for tool suffix (with Qwen3/3.5 type of chat templates)

[casinca]

What does this PR do?

Hi,
this should be a Fix #5317

I went with option 1 (model agnostic imho) mentioned by @mattbui in the issue, and mirrored it to the new exp async GRPO.
Generated a test case, in case.

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---

Note

Medium Risk
Changes how tool-result token suffixes are computed in both GRPOTrainer and experimental async GRPO; mistakes here could break tool-calling flows or cause subtle tokenization mismatches for certain chat templates.

Overview
Fixes tool-call prompt reconstruction for chat templates (notably Qwen3/Qwen3.5) that emit an EOS token followed by extra trailing tokens (e.g. newline). When deriving tool-result suffix_ids, the code now trims the prefix to the last EOS before slicing and validates the EOS-trimmed prefix matches the full tokenization, preventing incorrect suffix extraction.

Adds a regression test (test_get_tool_suffix_ids_eos_newline_boundary) covering Qwen3 and Qwen3.5 tokenizers (with version guard) to ensure _get_tool_suffix_ids returns the expected suffix under this EOS+newline boundary condition.

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[qgallouedec]

LGTM! thanks a lot for the fast fix!

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