StreamingResponse.EndStream returns result

[tracyboehrer]

Fixes #466

For discussion:

• Instead of throwing from EndStreamAsync, which would introduce new and unexpected behavior, EndStream will return a result.

public enum StreamingResponseResult
{
    Success,
    NotStarted,
    AlreadyEnded,
    UserCancelled,
    Timeout,
    Error
};

[Copilot]

Pull Request Overview

This PR changes the EndStreamAsync method to return a StreamingResponseResult enum instead of throwing exceptions, providing callers with explicit feedback about the streaming operation outcome.

Key Changes:

• Introduced StreamingResponseResult enum with values: Success, NotStarted, AlreadyEnded, UserCancelled, Timeout, and Error
• Modified EndStreamAsync to return Task<StreamingResponseResult> instead of Task
• Added comprehensive test coverage for various streaming scenarios including success, cancellation, fallback, and duplicate end calls

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
IStreamingResponse.cs	Added StreamingResponseResult enum and updated EndStreamAsync signature to return result
StreamingResponse.cs	Implemented result-based error handling in EndStreamAsync, added _userCanceled field, and improved error categorization logic
StreamingResponseTests.cs	Added new test file with comprehensive coverage of streaming scenarios including success cases, user cancellation, fallback behavior, and duplicate calls

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