LANGSMITH_TRACING environment variable prevents per-model tracing control

### Checked other resources - [x] This is a feature request, not a bug report or usage question. - [x] I added a clear and descriptive title that summarizes the feature request. - [x] I used the GitHub search to find a similar feature request and didn't find it. - [x] I checked the LangChain documentation and API reference to see if this feature already exists. - [x] This is not related to the langchain-community package. ### Package (Required) - [x] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [x] langchain-core - [ ] langchain-cli - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-perplexity - [ ] langchain-prompty - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Feature Description Currently, when LangSmith tracing is enabled globally using the LANGSMITH_TRACING environment variable, all LLM calls are traced unconditionally. Even when wrapping a call with traceable({ tracingEnabled: false }), the trace is still emitted. ### Use Case This feature allows tracing to be explicitly disabled for a specific LLM invocation, regardless of global tracing configuration. ### Proposed Solution While investigating the current implementation, I noticed that `tracing` is not passed into the `isTracingEnabled `check when the callback manager is configured in `libs/langchain-core/src/callbacks/manager.ts`. As a result, tracing behavior is effectively controlled only at the global/environment level, and per-call or per-instance overrides cannot take effect. I propose introducing a tracing configuration on the BaseChatModel and BaseLangChainParams, allowing tracing to be enabled or disabled at the model instance level. Example usage: ```ts const llm = new…