Migrate to langchain >=0.1.0,<0.3

[ehsandaya1-blip]

This updates the project's use of langchain from <0.1.0 to >=0.1.0,<0.3, following the provider's migration guide (https://python.langchain.com/docs/versions/v0_2/).

What changed

• langchain.chat_models.ChatOpenAI / langchain.llms.OpenAI / langchain.embeddings.OpenAIEmbeddings (+ Azure variants) → langchain_openai.{ChatOpenAI, OpenAI, OpenAIEmbeddings, AzureChatOpenAI, AzureOpenAI, AzureOpenAIEmbeddings} (13 sites): OpenAI integrations moved to the langchain-openai partner package.
• langchain.{vectorstores,document_loaders,utilities,tools,retrievers,chat_models,llms,embeddings,docstore,graphs,storage,agent_toolkits}.* → langchain_community.<same path> (6 sites): Third-party integrations moved to langchain-community with identical module paths.
• langchain.text_splitter → langchain_text_splitters (3 sites): Text splitters moved to their own package (0.2).

Files touched: 8.

How this was produced

Deterministic rewrites from a machine-readable MigrationSpec for this upgrade, then a review pass on the semantic residue; every changed file was parsed and checked with pyflakes for new undefined names before this PR was prepared. No behaviour beyond the migration was touched: no reformatting, no import reordering, no unrelated code.

Happy to adjust anything — if the project deliberately pins the old major version, say so and I will close this.

Notes for reviewers

• langchain.prompts, langchain.chains, from langchain import PromptTemplate / HuggingFaceHub still resolve in 0.1/0.2 and were left alone.