Go CI: unit tests, race-enabled checks, and cmd/internal structure

[Semih702]

What’s included

This PR introduces a proper Go quality gate for the project and aligns both
proxy and collector with industry-standard Go project structure.

✅ Code & Structure

• Split binaries into cmd/* and core logic into internal/*
• No behavior or feature changes in runtime logic
• Improved testability and maintainability

✅ Unit Tests

• Added unit tests for core logic (request handling, validation, helpers)
• Tests live under internal/* packages

✅ CI Quality Gate (PR blocking)

• gofmt formatting check
• go mod tidy + go mod verify
• go vet ./...
• go test -race ./... (race detector enabled)

CI runs per-module (proxy, collector) and remains fast enough for every PR.

Why

• Catch data races early in concurrency-heavy code paths
• Ensure consistent formatting and dependency hygiene
• Make future changes safer and easier to review

Notes

• All tests are executed with the Go race detector enabled
• No production behavior or API changes introduced