feat: Provide access to HTTP headers on connect or error

[jsonbailey]

Note

Provides header visibility for both successful connections and error responses.

• Adds on_connect callback in Client to receive case-insensitive HTTP response headers on each successful connection (including reconnects)
• Updates HTTPStatusError and HTTPContentTypeError to carry response headers; Client#connect now passes headers when raising these errors
• Initializes default no-op connect handler to maintain backward compatibility
• Adds spec/headers_spec.rb with end-to-end tests for header exposure on success, errors, reconnections, and backward compatibility

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