Add reader object access in callback for stream control

[Copilot]

This PR implements the feature request to allow access to the reader object in the fetchStreamedChat callback function, enabling users to cancel streams mid-processing.

Changes Made

Core Implementation

• Minimal Change: Modified the callback invocation in processStream to pass the reader object as a second parameter:

  // Before
  onChunkReceived(message);
  
  // After  
  onChunkReceived(message, reader);
  

New Usage Pattern

Users can now access the reader object to control the stream:

fetchStreamedChat({
    apiKey,
    messageInput: 'Hello, how are you?',
}, (responseChunk, reader) => {
    if (someCondition) {
        reader.cancel(); // Cancel the stream
    }
    
    const content = JSON.parse(responseChunk).choices[0].delta.content;
    if (content) {
        process.stdout.write(content);
    }
});

Backward Compatibility

Existing code continues to work unchanged:

// This still works exactly as before
fetchStreamedChat(options, (responseChunk) => {
    // handle chunk
});

Documentation & Testing

• Updated README.md with examples and documentation for the new feature
• Added test case to verify reader object is properly passed to callbacks
• Created demonstration scripts showing the exact requested usage pattern

Benefits

• ✅ Enables stream cancellation via reader.cancel()
• ✅ 100% backward compatible - no breaking changes
• ✅ Minimal implementation - only 1 line of functional code changed
• ✅ Addresses the exact use case described in the issue

Fixes #2.

[!WARNING]

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• api.openai.com
  • Triggering command: node /home/REDACTED/work/streamed-chatgpt-api/streamed-chatgpt-api/node_modules/.bin/jest test/reader-callback.test.js (dns block)
  • Triggering command: node test/compatibility-test.js (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

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