Client.Next cancellation via context + empty events fix

[x2d7]

Summary

This PR introduces context-aware cancellation to streaming and refines handling of empty chunks in the OpenAI connector. The Stream interface has been updated to accept context.Context in Next, enabling deterministic cancellation and improved control in concurrent scenarios.

Empty SSE chunks no longer produce artificial errors. Instead, they are skipped transparently, preventing premature stream termination.

The changes affect the chat core and the connect/openai implementation, with corresponding test updates.

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Changes

1. Stream interface update (breaking change)

type Stream[T any] interface {
    Next(ctx context.Context) bool
    Current() T
    Err() error
    Close() error
}

All implementations and call sites were updated accordingly.

2. OpenAIStream improvements

• Next(ctx) now checks ctx.Done() before attempting to fetch the next SSE chunk.
• On cancellation, the stream stops and sets s.err = ctx.Err().
• Empty parsed event batches are skipped recursively instead of treated as errors.
• Removed artificial "empty events" error from handleRawChunk.

This ensures:

• Correct cancellation semantics.
• No unexpected failures on valid but empty chunks.
• Cleaner streaming behavior.

3. Chat integration

Chat.Complete now propagates ctx into stream.Next(ctx), allowing external cancellation to stop streaming immediately.

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Tests

connect/openai

Removed

• TestHandleRawChunkDecorator_EmptyEvents_ReturnsError
• TestNext_EmptyChunk_StopsWithEmptyEventsError

These tests validated behavior that is no longer correct (empty chunks are not errors).

Added

• TestNext_ContextCancellation_StopsStream

Test	Expected Behavior
ContextCancellation	When ctx is canceled, Next(ctx) stops and returns context.Canceled

All related mocks were updated to support the new Next(ctx) signature.