fix: return read error when ErrorResponse has nil Error field

[hassan123789]

## Summary

Fixes #1060

When streaming is cancelled (e.g., via context cancellation), `stream.Recv()` returns `"error, <nil>"` instead of propagating the original read error (like `io.EOF`).

## Root Cause

In `stream_reader.go` line 66-68, when `ReadBytes('\n')` returns an error (like `io.EOF` on context cancellation), `unmarshalError()` is called. If the error accumulator contains an empty JSON object `{}`, it unmarshals to `&ErrorResponse{Error: nil}`. Since `respErr != nil` is true (the struct pointer exists), but `respErr.Error` is nil, `fmt.Errorf("error, %w", respErr.Error)` produces `"error, <nil>"`.

## Solution

Add a nil check for `respErr.Error`:

```go
if respErr != nil && respErr.Error != nil {
    return nil, fmt.Errorf("error, %w", respErr.Error)
}

This ensures the original read error is properly propagated when there's no actual API error to report.

Changes

• stream_reader.go: Added nil check for respErr.Error before formatting error message
• stream_reader_test.go: Added test case TestStreamReaderReturnsReadErrWhenErrorResponseHasNilError to verify the fix

Testing

All existing tests pass, and the new test verifies that io.EOF is properly returned instead of "error, <nil>".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.61%. Comparing base (5d7a276) to head (09600b0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1096      +/-   ##
==========================================
+ Coverage   99.59%   99.61%   +0.02%     
==========================================
  Files          34       34              
  Lines        2209     1841     -368     
==========================================
- Hits         2200     1834     -366     
+ Misses          6        4       -2     
  Partials        3        3              

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