fix: Semantic-embeddings stream errors treated as EOF, indexing partial summaries silently

[majiayu000]

Summary

This PR fixes #1105

Problem

The stream.Recv() loop in BuildEmbeddingInput was breaking on any error, treating real stream errors (network failures, context cancellations, API errors) the same as io.EOF. This caused partial/incomplete summaries to be silently indexed when stream errors occurred, degrading RAG retrieval quality.

Changes

• Added proper io.EOF check using errors.Is(err, io.EOF) to distinguish normal end-of-stream from real errors
• Return the error to caller on non-EOF errors instead of silently continuing with partial data
• Added necessary imports (errors, io)

Fix Pattern

The fix follows the established pattern used elsewhere in the codebase (e.g., pkg/runtime/runtime.go):

for {
    resp, err := stream.Recv()
    if errors.Is(err, io.EOF) {
        break
    }
    if err != nil {
        return "", fmt.Errorf("error receiving from semantic generation stream: %w", err)
    }
    // ... process response
}

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