feat: transparent request headers

[christiangda]

No description provided.

[Copilot]

Pull Request Overview

This PR implements transparent request header handling for the HTTP retry client, making it a true drop-in replacement for http.Client. It removes external dependencies, updates documentation to emphasize transparent operation, and simplifies the codebase.

• Removes testify dependency and replaces with custom helper functions
• Updates comments and documentation to emphasize transparent behavior
• Removes verbose debug output to make retries silent and transparent

Reviewed Changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
httpretrier.go	Removes debug print statement for silent retry operation
http_client_test.go	Replaces testify assertions with custom helper functions and simplifies test logic
http_client.go	Updates comments to emphasize transparent operation and improves code structure
go.mod	Removes testify dependency and updates Go version to 1.25.1
example_httpretrier_test.go	Adds comprehensive examples demonstrating transparent header preservation
README.md	Updates documentation to emphasize transparent behavior and adds authorization examples
.github/workflows/release.yml	Updates GitHub Actions to newer versions
.github/workflows/pr.yaml	Updates GitHub Actions to newer versions

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