[Bug] Source insights (transformations) silently truncated at Esperanto default max_tokens=850 — no way to override for openai_compatible / non-reasoning models

Environment: open-notebook 1.14.0 (v1-latest-single), esperanto 2.25.1, provider openai_compatible (vLLM, served model Qwen3.8, 256K context window).

Problem

Source insights (transformations) are silently cut off mid-sentence / mid-word. The truncation correlates directly with the transcript length — long sources get cut, short ones complete cleanly:

Root cause

Transformations provision the language model via provision_langchain_model() (open_notebook/ai/provision.py) -> model.to_langchain(). No explicit max_tokens is passed anywhere in this path.

Esperanto's LanguageModel base default is max_tokens = 850 (esperanto/providers/llm/base.py:19). The OpenAI provider only skips this default cap for reasoning models (_is_reasoning_model() = name starts with o1/o3/o4/gpt-5, esperanto/providers/llm/openai.py:316). An openai_compatible model such as Qwen3.8 is not detected as reasoning, so every transformation request is sent with max_tokens=850.

There is no way to override this from Open Notebook:

• ModelCreate / ModelResponse have no max_tokens field.
• Credential.to_esperanto_config() only emits known keys (api_key, base_url, endpoint*, num_ctx, ...) — arbitrary keys like max_tokens are dropped.
• No relevant env var / setting exists.

Verified live in the container:

m = await model_manager.get_default_model("transformation")   # OpenAICompatibleLanguageModel "Qwen3.8"
m.max_tokens                 # -> 850
m.to_langchain().max_tokens  # -> 850  (ChatOpenAI)

The backend model itself is not the limit — a direct request to the same endpoint with max_tokens=3000 returns a complete 2852-token answer (finish_reason=stop). So the cap is imposed solely by Open Notebook/Esperanto.

Suggested fix

Analogous to the Ask fix in #1241 (OPEN_NOTEBOOK_ASK_MAX_TOKENS): make the transformation output budget configurable — e.g. an OPEN_NOTEBOOK_TRANSFORMATION_MAX_TOKENS env var and/or a per-model max_tokens in the model/credential config — and/or stop silently falling back to Esperanto's 850 default for non-reasoning openai_compatible models.

Related

#1221, #1247, #1241 (Ask/Q&A truncation), #1200 (provider-declared tunable params), #947 (model & provider advanced config).