Add unit tests and GitHub Actions

[google-labs-jules]

Added comprehensive unit tests for configuration, handlers, and proxy logic. Fixed compilation errors in MITM proxy implementation. Set up GitHub Actions for continuous integration.

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PR created automatically by Jules for task 9624064502948816532 started by @nshmdayo

[google-labs-jules]

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[nshmdayo]

@jules what are config_test.go and handler_test.go?

[google-labs-jules]

@jules what are config_test.go and handler_test.go?

These are the new unit test files added in this PR. app/config/config_test.go contains tests for the LoadConfig function to verify that configuration files are parsed correctly. app/handlers/handlers_test.go contains tests for the ModificationHandler and LoggingHandler functions to ensure they modify requests/responses and log information as expected.

[nshmdayo]

@jules Why are you changing mitm.go?

[google-labs-jules]

Why are you changing mitm.go?

I modified mitm.go to fix a compilation error found during testing. The code was attempting to pass a *tls.Conn to server.Serve, but server.Serve expects a net.Listener. I introduced a helper struct singleConnListener to wrap the tls.Conn and satisfy the net.Listener interface, ensuring server.Serve works correctly with the hijacked connection.