pr/refactor getbody retry ci

[KingPin]

• fix(sendRequest): handle nil GetBody, close response on retry, and constant-format logging
• test: use closed localhost port for deterministic network-error test
• docs: add copilot instructions and docker-only + atomic commits guidance
• refactor: pre-read request body when GetBody missing; improve retry detection (net/url errors)

[Copilot]

Pull request overview

This PR improves request retry robustness in the FanOut Go service by handling GetBody being nil, tightening retryable-network-error detection, and making CI/tests more deterministic, with accompanying documentation updates.

Changes:

• Update multiplex/sendRequest to better support retries when GetBody is nil (pre-read body) and to close responses on retry.
• Improve retryability detection using typed net.Error / url.Error checks and adjust logging calls to use constant formats.
• Add/adjust tests for deterministic network errors and GetBody-nil scenarios; update Docker CI steps and add repo Copilot instructions.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
fanout.go	Adds body pre-read fallback when GetBody is nil; improves retry error detection and retry cleanup; adjusts logging usage.
fanout_test.go	Makes the network-error test deterministic by using a closed localhost port.
fanout_additional_test.go	Adds tests for multiplex handling without GetBody and for retryable net.Error.
.github/workflows/docker-image.yml	Runs formatting/vet, gosec, and tests inside Docker in CI.
.github/copilot-instructions.md	Documents repo-specific build/test conventions and workflow preferences.

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[Copilot]

bodyReader can be nil when getBody == nil, but on http.NewRequestWithContext error you unconditionally call bodyReader.Close(), which will panic. Guard the close with a nil check (or avoid closing here and rely on Transport when Do is called).

Suggested change

	bodyReader.Close()
	if bodyReader != nil {
	bodyReader.Close()
	}

[Copilot]

This step masks failures: go vet ./... || true will always succeed even when vet finds issues, and gofmt -l . won’t fail the job when files are unformatted. Make the step fail on vet/gofmt findings (e.g., remove || true for vet and explicitly error when gofmt -l outputs any files).

Suggested change

	docker run --rm -v ${{ github.workspace }}:/src -w /src golang:1.24 sh -c "gofmt -l . || true; go vet ./... || true"
	docker run --rm -v ${{ github.workspace }}:/src -w /src golang:1.24 sh -c 'set -e
	fmt_output=$(gofmt -l .)
	if [ -n "$fmt_output" ]; then
	echo "gofmt found unformatted files:"
	echo "$fmt_output"
	exit 1
	fi
	go vet ./...'

[Copilot]

This test claims http.NewRequest leaves GetBody nil, but for common in-memory body types like *bytes.Reader Go sets GetBody automatically. As written, this may not exercise the getBody == nil branch in multiplex; consider explicitly clearing req.GetBody or using a body type that doesn’t trigger automatic GetBody generation.

Suggested change

	// Create a request WITHOUT GetBody (http.NewRequest leaves GetBody nil)
	body := []byte("hello")
	req, err := http.NewRequest("POST", "/fanout", bytes.NewReader(body))
	if err != nil {
	t.Fatalf("Failed to create request: %v", err)
	}
	// Create a request and explicitly clear GetBody to simulate a request without it
	body := []byte("hello")
	req, err := http.NewRequest("POST", "/fanout", bytes.NewReader(body))
	if err != nil {
	t.Fatalf("Failed to create request: %v", err)
	}
	req.GetBody = nil // ensure GetBody is nil so multiplex exercises the no-GetBody path