Add file part support to chat message structure

[alejandrojnm]

Introduces ChatMessageFile struct and ChatMessageTypeFile constant to support file attachments in chat messages. Updates ChatMessagePart to include file parts and adds comprehensive tests for serialization, deserialization, and constant definitions.

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There is one https://github.com/sashabaranov/go-openai/pull/1053/files but this have the test

Describe the change
This PR is to add file to the message

Provide OpenAI documentation link
https://platform.openai.com/docs/api-reference/chat/create

Describe your solution
The solution is simple, there is not much to explain

Tests
Test are include

[alejandrojnm]

HI @sashabaranov you think we can put this one in the next release ?

[Copilot]

Pull Request Overview

This PR adds support for file attachments in chat messages by introducing a new ChatMessageFile struct and ChatMessagePartTypeFile constant. This enables the OpenAI Go client to handle file parts within chat message structures.

• Introduces ChatMessageFile struct with fields for file ID, filename, and base64-encoded file data
• Adds ChatMessagePartTypeFile constant and updates ChatMessagePart to include file support
• Comprehensive test coverage for serialization, deserialization, and constant validation

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
chat.go	Defines ChatMessageFile struct, adds ChatMessagePartTypeFile constant, and updates ChatMessagePart to include File field
chat_test.go	Adds comprehensive tests for file part functionality including serialization/deserialization and constant validation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.59%. Comparing base (c4273cb) to head (fa78712).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1056   +/-   ##
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  Coverage   99.59%   99.59%           
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  Files          34       34           
  Lines        2205     2205           
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  Hits         2196     2196           
  Misses          6        6           
  Partials        3        3           

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