test: Add comprehensive unit tests for Go SDK SSE client

- Added extensive test coverage for SSE client streaming functionality
- Coverage improved from 12.5% to 97.1% for the SSE client package
- Tests cover authentication, error handling, context cancellation,
  multiline data, concurrent operations, and timeout scenarios
- Added performance benchmarks for streaming operations
- Overall SDK test coverage now at 82.6%

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: mattsp1290