docs: updated readmefile to support latest langchain version

[vkpatva]

Description:

This PR updates the README.md quick-start example to use the current LangChain v1.0.1+ API in order to ensure the documentation matches working code implementations and provides accurate examples for new users.

The previous example used deprecated LangChain patterns that no longer work with current versions. This update aligns the documentation with the actual working implementation and maintains backward compatibility with multiple AI providers.

• Replace createToolCallingAgent with createOpenAIToolsAgent for OpenAI models
• Add conditional logic to use createOpenAIToolsAgent for OpenAI and createToolCallingAgent for other providers (Claude, Groq, Ollama)
• Update ChatOpenAI initialization to explicitly use modelName and openAIApiKey parameters
• Remove chat_history placeholder from prompt template to match current working demo structure
• Add verbose: true to AgentExecutor for better debugging output
• Support both ACCOUNT_ID/PRIVATE_KEY and HEDERA_ACCOUNT_ID/HEDERA_PRIVATE_KEY environment variable naming conventions
• Add await keyword for agent creation (required in LangChain v1.0.1+)
• Update prompt structure to match current LangChain API patterns

Notes for reviewer:

The changes have been tested against a working implementation and verified to match the current LangChain API. The example code:

• Works with OpenAI using createOpenAIToolsAgent
• Maintains compatibility with Claude, Groq, and Ollama using createToolCallingAgent
• Supports flexible environment variable naming
• Follows the same structure as the working demo in the codebase

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.) - Verified against working implementation