refactor: refactor HTTP client for improved context and error handling

[appleboy]

• Replace defer-based file close with explicit file.Close() calls for error handling in large file upload example
• Update tests to use NewRequestWithContext for improved context control
• Refactor retry logic into smaller functions for maintainability and clarity
• Use a single contentTypeJSON constant in place of repeated string literals in tests
• Improve context cancellation handling during retries and response body closing
• Consolidate per-attempt metrics and status recording in HTTP retries

[Copilot]

Pull request overview

This PR refactors the HTTP client implementation to improve code maintainability, context handling, and error handling patterns. The changes focus on extracting retry logic into smaller, more focused functions and updating tests to use more explicit context management.

Changes:

• Refactored retry logic in retry.go into three smaller functions (calculateRetryDelay, executeAttempt, wrapBodyWithCancel) for improved maintainability
• Updated all test files to use http.NewRequestWithContext instead of http.NewRequest for better context control
• Introduced contentTypeJSON constant in retry_test.go to eliminate string literal duplication
• Modified example_test.go to use explicit file.Close() calls instead of defer for clearer error handling

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
retry.go	Refactored DoWithContext retry logic into three smaller helper functions (calculateRetryDelay, executeAttempt, wrapBodyWithCancel) with corresponding result structs for improved code organization and testability
tracer_test.go	Updated test to use NewRequestWithContext for improved context control
retry_test.go	Added contentTypeJSON constant and replaced all "application/json" string literals; consistent with context-aware request creation
observability_test.go	Updated all tests to use NewRequestWithContext for better context management
metric_test.go	Updated tests to use NewRequestWithContext and added defensive response body closing for error cases
example_test.go	Changed from defer-based file closing to explicit file.Close() calls in error paths for more granular error handling control

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[Copilot]

If client.Do returns an error with a non-nil response, the response body should be closed before calling log.Fatal to avoid resource leaks. Consider adding:

if resp != nil && resp.Body != nil {
    resp.Body.Close()
}

before the log.Fatal call.