fix(reasoning): append promoted tool calls in qwen3 parser, not prepe…

[nac7]

Fixes #39056

Summary

When Qwen3.5 emits XML tool calls inside <think>, the reasoning parser
promotes them into content so qwen3_coder can parse them.

PR #39055 prepended the promoted blocks, which caused Qwen3CoderToolParser
to discard any pre-existing response text — it extracts content up to the
first <tool_call> marker, so prepending put existing text after the
marker where it gets silently dropped.

Changes

• vllm/reasoning/qwen3_reasoning_parser.py: append promoted tool-call
  blocks after existing content instead of prepending them
• tests/reasoning/test_qwen3_tool_call_recovery.py: adds the missing
  assertion flagged by the Gemini reviewer in Fix Qwen3 reasoning tool calls embedded inside think #39055 — verifies that
  trailing response text survives the full parser pipeline end-to-end

Before / After

Scenario	Before	After
Tool call inside <think>, no trailing text	❌ tool call lost	✅ recovered
Tool call inside <think> + trailing text	❌ trailing text discarded	✅ both preserved
Normal reasoning, no tool call	✅ unchanged	✅ unchanged

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