Optimize Streaming Response Handling

[shroominic]

Ports the streaming optimization and X-Cashu refactoring from commit d8a81fb (originally in PR #269).

Changes:

• Implements tail buffering for streaming responses to optimize usage extraction.
• Refactors X-Cashu handlers to buffer streaming responses, allowing refund headers to be calculated and sent before the body stream begins.
• Adds helper method.