Support vLLM 0.20.2 / NIM reasoning-parser output end-to-end (surface + agent context + render) (#602)

* fix(stream): read 'reasoning' SSE field for vLLM 0.20.2 / NIM

vLLM 0.20.2 / NVIDIA NIM emit reasoning-parser output in the `reasoning` delta field; older builds use `reasoning_content`. stream_llm() read only the latter, so reasoning from models like Nemotron-3-Nano (--reasoning-parser) was silently dropped and never rendered. Accept either field.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(agent): keep reasoning_content only on the latest assistant turn

The agent loop echoed each round's reasoning back as `reasoning_content` on every assistant turn, assuming vendors ignore it. Nemotron's chat template re-injects ALL prior reasoning_content as <think> blocks, and the loop is trimmed only once (before it starts) — so reasoning accumulated unbounded across rounds, bloating context and feeding the model its own prior reasoning, which reinforced repetition/looping. Strip reasoning_content from earlier assistant turns so only the most recent round carries it (still satisfies DeepSeek's thinking-mode follow-up requirement).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(agent-ui): wrap each round's reasoning in its own <think> block

The streamed think-tag wrapper gated on whole-message substring checks (accumulated.includes('<think>')), which only ever wrapped ONE reasoning block per message. A multi-round agent response has a reasoning phase per round, so once round 1 closed its <think>...</think>, rounds 2+ reasoning was emitted unwrapped and leaked into the visible answer. Replace the substring checks with a stateful open/close flag that toggles per think/answer cycle, so each round's reasoning gets its own collapsible block. Single-turn chat is unchanged (one open, one close).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* test(stream): reasoning/reasoning_content delta surfaces as thinking chunk

Covers @pewdiepie-archdaemon's requested regression: a streamed {reasoning: ...} delta emits a thinking chunk while {content: ...} streams as normal content; plus the older reasoning_content field for backward compat. Mirrors the #591 scenario.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/llm_core.py

@@ -1127,8 +1127,8 @@ async def stream_llm(url: str, model: str, messages: List[Dict], temperature: fl
                                     delta = j["choices"][0].get("delta") or {}
                                     if isinstance(delta, dict):
                                         # Text content
-                                        # Reasoning tokens (VLLM --reasoning-parser, e.g. Qwen3/DeepSeek-R1)
-                                        reasoning = delta.get("reasoning_content") or ""
+                                        # Reasoning tokens (VLLM --reasoning-parser, e.g. Qwen3/DeepSeek-R1, Nemotron). vLLM 0.20.2 / NIM emit the field as `reasoning`; older builds use `reasoning_content`. Accept either.
+                                        reasoning = delta.get("reasoning_content") or delta.get("reasoning") or ""
                                         if reasoning:
                                             yield f'data: {json.dumps({"delta": reasoning, "thinking": True})}\n\n'
                                         content = delta.get("content") or ""