fix(openai): increase SSE scanner buffer to 1MB and remove duplicate error check (#72)

Default bufio.Scanner limit (64KB) can truncate large SSE lines from
tool call arguments or thinking content, causing silent parse failures.
Set buffer to 1MB to match the scale of extended thinking responses.

Also remove a duplicate scanner.Err() check that was accidentally left in.

internal/providers/openai.go

@@ -103,6 +103,7 @@ func (p *OpenAIProvider) ChatStream(ctx context.Context, req ChatRequest, onChun
 	accumulators := make(map[int]*toolCallAccumulator)
 
 	scanner := bufio.NewScanner(respBody)
+	scanner.Buffer(make([]byte, 0, 64*1024), 1024*1024) // 1MB max line for large tool call / thinking chunks
 	for scanner.Scan() {
 		line := scanner.Text()
 		if !strings.HasPrefix(line, "data:") {
@@ -185,11 +186,6 @@ func (p *OpenAIProvider) ChatStream(ctx context.Context, req ChatRequest, onChun
 		return nil, fmt.Errorf("%s: stream read error: %w", p.name, err)
 	}
 
-	// Check for scanner errors (timeout, connection reset, etc.)
-	if err := scanner.Err(); err != nil {
-		return nil, fmt.Errorf("%s: stream read error: %w", p.name, err)
-	}
-
 	// Parse accumulated tool call arguments
 	for i := 0; i < len(accumulators); i++ {
 		acc := accumulators[i]