Avoid hard dependency on tiktoken (make it optional / lazy-loaded)

### Submission checklist - [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) - [ ] langchain - [x] langchain-openai - [x] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] 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-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Feature Description `tiktoken` is valuable for OpenAI-compatible models, but it should be treated as a provider-specific implementation detail, not a core dependency. In our case, it is unclear where and how `tiktoken` is being used across LangChain, which makes it difficult to safely remove or replace. This creates friction for production adoption, especially in environments where dependencies must be tightly controlled. We would prefer not to include `tiktoken`, but the current implicit usage makes it hard to assess the impact and safely opt out. It would be helpful if the dependency on `tiktoken` were more explicit, optional, and isolated to OpenAI-specific integrations. ### Use Case ### 1. Not compatible with multi-model usage Different providers have different tokenization: * Anthropic * Gemini * Open-source models (Llama, Mistral, etc.) `tiktoken` is OpenAI-specific, but is effectively treated as a default. --- ### 2. Issues in production environments In local/on-prem deployments: * model pr…