fix(middleware): halt execution chain after writing error response

[s6pa1rta3n-lab]

Summary of Changes

• Implemented robust HTTP middleware patterns with explicit early returns after writing headers or invoking http.Error.
• Prevents superfluous response.WriteHeader warnings and stops execution from falling through to downstream handlers.
• Added comprehensive unit tests in middleware/auth_test.go verifying status codes, response headers, and ensuring spy downstream handlers are completely bypassed on rejected requests.

Closes #1
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