fix: preserve respond_with_error payloads

[bougyman]

Summary

• stop coercing respond_with_error payloads to strings before handing them to nats-pure
• forward blocks to the underlying NATS message so { code: 422, ... } payloads can be adjusted in place
• preserve raised exceptions as exception objects and add regression coverage for hash failures and block passthrough

Testing

• bundle exec rake test

Closes #39

[Copilot]

Pull request overview

This PR fixes respond_with_error behavior so error payloads (e.g., hashes with custom code) and raised exceptions are preserved as-is when passed through to nats-pure, enabling non-500 error codes and in-place adjustments via blocks.

Changes:

• Stop coercing respond_with_error payloads to strings; pass objects (including Exceptions and Hashes) through unchanged.
• Forward blocks from MessageWrapper#respond_with_error to the underlying NATS message error response.
• Add regression tests covering exception passthrough, hash failure passthrough, and block forwarding.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
lib/leopard/message_wrapper.rb	Removes string coercion and forwards optional blocks when responding with errors.
lib/leopard/nats_api_server.rb	Preserves raised exceptions by passing the exception object (not its message) to respond_with_error.
test/lib/message_wrapper.rb	Adds/updates tests to assert exceptions/hashes are passed through and blocks are forwarded.
test/lib/nats_api_server.rb	Updates handler-raise test to assert exception object passthrough and adds hash-failure regression coverage.

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